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PubMed1982

Cytoplasmic male sterility in barley: Evidence for the involvement of cytokinins in fertility restoration.

The hypothesis of the association between an increase in cytokinin activity and restoration of anther fertility in msm1 cytoplasm was tested. The following barley lines with Hordeum vulgare cv. Adorra nuclear gene background were studied: Adorra cytoplasm without nuclear restorer gene (fertile), Adorra cytoplasm homozygous for nuclear Rfm1a gene (fertile), msm1 cytoplasm without restorer gene (male sterile), msm1 cytoplasm homozygous for nuclear Rfm1a gene (fertile). Ethanolic extracts of root exudate were fractionated and bioassayed for cytokinins. Both the biological activity and the total quantity of cytokinins appeared lowest in the unrestored male sterile line. The total biological activities of cytokinins in the three fertile lines were similar, but the quantities in the restorer gene carriers appeared lower. On the other hand, the restorer gene carriers, independent of the cytoplasm, showed 8-9 times more of a bound cytokinin. Because the bound form is evidently underestimated by the bioassay, the increase in the bound cytokinin fraction may mean even a higher total content in the Rfm1a gene carriers than in Adorra without the gene. The bound cytokinin may be translocated more readily to distal organs (e.g., the anthers) compared with unbound cytokinins. Because cytokinins are associated with various ecophysiological processes, the rise in a particular form may explain the heterogeneous distribution of the restorer gene in wild barley populations in different regions of Israel.

Journal Article↗
PubMed1987

Mutation to male fertility and toxin insensitivity in Texas (T)-cytoplasm maize is associated with a frameshift in a mitochondrial open reading frame.

Tissue culture-derived mutants of male-sterile and disease toxin-sensitive Texas (T)-cytoplasm maize that exhibit male fertility and toxin insensitivity carry numerous alterations in mitochondrial DNA. In these mutants, a 6.7-kilobase Xho I fragment characteristic of parental T cytoplasm has been rearranged. In the mutant T-4, the parental 6.7-kilobase Xho I fragment contains a guanine to adenine transition adjacent to a 5-base-pair insertion not found in T cytoplasm. The insertion, internal to a 345-base-pair open reading frame (T ORF13), generates a frameshift, resulting in a premature stop codon that terminates the open reading frame at base pair 222. In other mutants, the 345-base-pair ORF is part of a 3-kilobase deletion, which extends into a 5-kilobase repeat characteristic of mtDNA from T but not N male-fertile cytoplasm. Clones specific to T ORF13 hybridize to eight transcripts in T and T-4, yet only hybridize to three transcripts in T-7, a deletion mutant. Transcription of the T ORF13 region appears not to be altered in T-4, but the frameshift mutation in the T ORF13 reading frame indicates that a biologically inactive gene product could be associated with the mutational events. The results suggest that cytoplasmic male sterility and disease toxin sensitivity may be associated with presence of T ORF13 in T-cytoplasm maize.

Journal Article↗
PubMed1997

Dissecting the thrombopoietin receptor: functional elements of the Mpl cytoplasmic domain.

Thrombopoietin (TPO) acts through its receptor, Mpl, to stimulate the proliferation and maturation of megakaryocytes and their progenitors. The Mpl cytoplasmic domain controls this process through assembly of an active signaling complex using various receptor docking sites. In this report, eight carboxyl truncations of the 121-aa murine Mpl cytoplasmic domain were tested for the ability to support growth of a cytokine-dependent cell line (Ba/F3) and for their capacity to induce TPO-stimulated tyrosine phosphorylation of specific signaling proteins. Point mutations of the five tyrosine residues in the cytoplasmic domain of the receptor were subsequently used to confirm our conclusions. From these studies we demonstrate that: (i) TPO-induced proliferation is moderately reduced by truncation of as many as 53 C-terminal amino acids of Mpl, including the sites of receptor tyrosine phosphorylation; (ii) truncation/mutation of residues 69-83 of the Mpl cytoplasmic domain enhances proliferative signaling, perhaps mediated by a decrease in receptor-driven cellular differentiation; (iii) Mpl can be phosphorylated at either Y112 or Y117 but not at the three proximal cytoplasmic tyrosine residues (Y8, Y29, and Y78); (iv) Y112 of Mpl is necessary for tyrosine phosphorylation of Shc and Shc-associated p145 (SHIP); and (v) unlike STAT3, STAT5 is partially phosphorylated in the absence of any tyrosine residues in the Mpl cytoplasmic domain. These studies identify subdomains of Mpl necessary for activation of several critical signaling pathways and point to two potentially novel mechanisms of TPO-induced signal transduction, an indirect pathway to STAT5 activation and a differentiation domain that acts by limiting proliferation.

Animals↗
PubMed2000

The long cytoplasmic tail of gp41 is required in a cell type-dependent manner for HIV-1 envelope glycoprotein incorporation into virions.

Lentiviruses, including HIV-1, have transmembrane envelope (Env) glycoproteins with cytoplasmic tails that are quite long compared with those of other retroviruses. However, mainly because of the lack of biochemical studies performed in cell types that are targets for HIV-1 infection, no clear consensus exists regarding the function of the long lentiviral Env cytoplasmic tail in virus replication. In this report, we characterize the biological and biochemical properties of an HIV-1 mutant lacking the gp41 cytoplasmic tail. We find that the gp41 cytoplasmic tail is necessary for the efficient establishment of a productive, spreading infection in the majority of T cell lines tested, peripheral blood mononuclear cells, and monocyte-derived macrophages. Biochemical studies using a high-level, transient HIV-1 expression system based on pseudotyping with the vesicular stomatitis virus glycoprotein demonstrate that in HeLa and MT-4 cells, mutant Env incorporation into virions is reduced only 3-fold relative to wild type. In contrast, gp120 levels in virions produced from a number of other T cell lines and primary macrophages are reduced more than 10-fold by the gp41 truncation. The Env incorporation defect imposed by the cytoplasmic tail truncation is not the result of increased shedding of gp120 from virions or reduced cell-surface Env expression. These results demonstrate that in the majority of T cell lines, and in primary cell types that serve as natural targets for HIV-1 infection in vivo, the gp41 cytoplasmic tail is essential for efficient Env incorporation into virions.

Cell Line↗
PubMed1996

Heat shock protein 84 forms a complex with mutant p53 protein predominantly within a cytoplasmic compartment of the cell.

Cellular DNA damage results in the increased expression and accumulation of the p53 tumor suppressor protein within the nucleus which leads to cell cycle arrest or apoptosis. In some cases, however, wild-type p53 and some mutant forms of p53 reside in the cytoplasm of cancer cells. To understand the mechanism responsible for its cytoplasmic retention, studies were undertaken to determine if unique proteins form a complex with mutant p53 within the cytoplasm of transformed cells. One protein, with an apparent molecular mass of 92 kDa (p92), was observed to form a complex with a temperature-sensitive mutant p53 (TSp53(Val-135)) in the cytoplasm of transformed rat embryo fibroblasts at the non-permissive temperature. p92 copurified with TSp53(Val-135) on a p53-specific immunoaffinity column and a gel filtration column. The protein was purified to homogeneity and identified as hsp84 by partial amino acid sequence analysis. hsp84 is a member of the hsp90 class of proteins. At the non-permissive temperature, TSp53(Val-135) and hsp84 colocalized in the cytoplasm near the nuclear envelope. At the permissive temperature, TSp53(Val-135) resides in the nucleus and expresses a "wild-type like" conformation. Under these conditions hsp84 continued to reside in the cytoplasm and little or no hsp84 formed a complex with p53. The results suggest that hsp84 binds mutant p53 in a spatial and/or conformation dependent manner.

Amino Acid Sequence↗
PubMed1998

Phosphorylation of the cytoplasmic tail of syndecan-4 regulates activation of protein kinase Calpha.

Syndecans are transmembrane proteoglycans capable of carrying both heparan and chondroitin sulfate chains. The cytoplasmic tail of syndecan-4 was recently reported to undergo in vivo phosphorylation on Ser183 in the membrane-proximal part of the tail (Horowitz, A., and Simons, M. (1998) J. Biol. Chem. 273, 10914-10918). However, the functional consequences of this event remain unknown. The cytoplasmic tail of syndecan-4 is known to undergo multimerization and to activate protein kinase Calpha (PKCalpha), with both events depending on the presence of the commonly occurring phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). In the present investigation we found that phosphorylation of Ser183 produced a 10-fold reduction in the ability of syndecan-4 to activate PKCalpha, without affecting its ability to bind the PKC. Because Ser183 is adjacent to positively charged lysine groups that resemble PIP2-binding regions in several other proteins, phosphorylation of this serine may affect the binding affinity of the syndecan-4 cytoplasmic tail to PIP2. We found that the Ser183-phosphorylated cytoplasmic tail of syndecan-4 has indeed a significantly lower affinity to PIP2 compared with the nonphosphorylated tail. Furthermore, Ser183 phosphorylation abolished PIP2-dependent oligomerization of syndecan-4 cytoplasmic tails. We conclude that Ser183 phosphorylation regulates syndecan-4-dependent activation of PKCalpha by reducing the affinity to PIP2 and inhibiting the oligomerization of syndecan-4 cytoplasmic tails. These results further support the role of syndecan-4 in signal transduction in endothelial cells.

Animals↗
PubMed2002

Phage display based cloning of proteins interacting with the cytoplasmic tail of membrane immunoglobulins.

The reduced quantity and quality of serum immunoglobulins (sIgs) in mutant mice expressing truncated cytoplasmic tails of IgE and IgG1 indicate an active role for the cytoplasmic domains of mIgG1 and mIgE. We used phage display technology to identify candidate proteins able to interact with the cytoplasmic tail of mIgE. Using a murine cDNA B cell library displayed on the surface of phage as prey and the 28 amino acid long cytoplasmic tail of IgE as bait, we isolated phage encoding the murine hematopoietic progenitor kinase 1 (HPK1). Surface plasmon resonance analysis measurements confirmed affinity of HPK1 to the mIgE cytoplasmic tail and revealed association to other immunoglobulin isotypes as well. Immunoprecipitation experiments, using lysates from two B cell lines expressing nitrophenyl (NP) specific mIgE molecules showed co-precipitation of IgE and HPK1. The interaction of HPK1 with the cytoplasmic domains of membrane immunoglobulins indicate an active role of the tails as part of an isotype specific signal transduction, independent from the Igalpha/Igbeta heterodimers, and may represent a missing link to upstream regulatory elements of HPK1 activation.

Amino Acid Sequence↗
PubMed1993

Functional role of the cytoplasmic domain of the integrin alpha 5 subunit.

The purpose of this study was to explore the functional role of the cytoplasmic domain of the alpha subunit of the alpha 5/beta 1 integrin, a fibronectin receptor. Mutant CHO cells that express very low levels of endogenous hamster alpha 5 subunit (CHO clone B2) were transfected with an expression vector containing full-length or truncated human alpha 5 cDNAs to form chimeric human alpha 5/hamster beta 1 integrins. Three transfectants were examined: B2a27 expresses a full-length human alpha 5 subunit with 27 amino acids in the cytoplasmic domain; B2a10 expresses an alpha 5 with a 17-amino acid cytoplasmic truncation; B2a1 expresses an alpha 5 with a 26-amino acid truncation. Levels of alpha 5/beta 1 surface expression in B2a27 and B2a10 cells were similar to that in wild type CHO cells. The expression of alpha 5/beta 1 in B2a1 cells was less, amounting to 15-20% of WT levels, despite message levels that were three to five times greater than those of B2a27. The transfectants were used to examine the role of the alpha 5 cytoplasmic domain in cell adhesion, cell motility, cytoskeletal organization, and integrin-mediated tyrosine phosphorylation. The adhesion characteristics of B2a27 and B2a10 cells on fibronectin substrata were similar to each other and to wild type CHO cells. B2a1 cells displayed slight reductions in the strength and rate of adhesion to fibronectin. Cell motility in the presence of fibronectin was similar for B2a27, B2a10, and wild type CHO cells, while the B2a1 cells were substantially less motile. Comparable degrees of cell spreading and extensive organization of actin filaments were observed for B2a27, B2a10, and wild type CHO cells on fibronectin substrata. The B2a1 cells spread to a lesser degree, and some organization of actin was observed; the untransfected B2 cells remained round on fibronectin substrata and showed no actin reorganization. Since the reduced motility and cell spreading observed in the B2a1 cells might be due either to reduced surface expression of alpha 5/beta 1 or to the truncation in the alpha 5 cytoplasmic domain, we used flow cytometric cell sorting to select populations of B2a1 and B2a27 cells expressing similar levels of cell surface alpha 5. The deficits in spreading and motility were present in B2a1 cells expressing high levels of alpha 5. Thus the region of the alpha 5 cytoplasmic domain adjacent to the membrane seems to play an important role in cytoskeletal organization and cell motility. We also examined whether alpha subunit truncation would affect integrin-mediated tyrosine phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1978

Evidence that biosynthesis of phosphatidylethanolamine, phosphatidylcholine, and triacylglycerol occurs on the cytoplasmic side of microsomal vesicles.

Experiments were performed to localize the hepatic microsomal enzymes of phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol biosynthesis to the cytoplasmic or lumenal surface of microsomal vesicles. Greater than 90 percent of the activities of fatty acid-CoA ligase (EC 6.2.1.3), sn-glycerol 3-phosphate acyltransferase (EC 2.3.1.15), lysophosphatidic acid acyltransferase, diacylglycerol acyltransferase (EC 2.3.1.20), diacylglycerol cholinephosphotransferase (EC 2.7.8.2), and diacylglycerol ethanolaminephosphotransferase (EC 2.7.8.1) was inactivated by proteolysis of intact microsomal vesicles. The phosphatidic acid phosphatase (EC 3.1.3.4) was not inactivated by any of the protease tested. Under conditions employed, <5 percent of the luminal mannose-6-phosphatase (EC 3.1.3.9) activity was lost. After microsomal integrity was disrupted with detergents, protease treatment resulted in a loss of >74 percent of the mannose-6-phosphatase activity. The latency of the mannose-6-phosphatase activity was not affected by protease treatment. Mannose-6-phosphatase latency was not decreased by the presence of the assay components of several of the lipid biosynthetic activities, indicating that those components did not disrupt the microsomal vesicles. None of the lipid biosynthetic activities appeared latent. The presence of a protease-sensitive component of these biosynthetic activities on the cytoplasmic surface of microsomal vesicles, and the absence of latency for any of these biosynthetic activities suggest that the biosynthesis of phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol occurs asymmetrically on the cytoplasmic surface of the endoplasmic reticulum. The location of biosynthetic activities within the transverse plane of the endoplasmic reticulum is of particular interest for enzymes whose products may be either secreted or retained within the cell. Phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol account for the vast majority of hepatic glycerolipid biosynthesis. The phospholipids are utilized for hepatic membrane biogenesis and for the formation of lipoproteins, and the triacylglycerols are incorporated into lipoproteins or accumulate within the hepatocyte in certain disease states (14). The enzymes responsible for the biosynthesis of these glycerolipids (Scheme I) from fatty acids and glycerol-3P have all been localized to the microsomal subcellular fraction (12, 16, 29, 30). Microsomes are derived from the endoplasmic reticulum and are sealed vesicles which maintain proper sidedness. (11, 22). The external surface of these vesicles corresponds to the cytoplasmic surface of the endoplasmic reticulum. Macromolecules destined for secretion must pass into the lumen of the endoplasmic reticulum (5, 23). Uncharged molecules of up to approximately 600 daltons are able to enter the lumen of rat liver microsomes, but macromolecules and charged molecules of low molecular weight do not cross the vesicle membrane (10, 11). Because proteases neither cross the microsomal membrane nor destroy the permeability barrier of the microsomal vesicles, only the enzymes and proteins located on the cytoplasmic surface of microsomal vesicles are susceptible to proteolysis unless membrane integrity is disrupted (10, 11). By use of this approach, several enzymes and proteins have been localized in the transverse plane of microsomal membranes (11). With the possible exception of cytochrome P 450, all of the enzymes and proteins investigated were localized asymmetrically by the proteolysis technique (11). By studies of this type, as well as by product localization, glucose-6-phosphate (EC 3.1.3.9) has been localized to the luminal surface of microsomal vesicles (11) and of the endoplasmic reticulum (18, 19). All microsomal vesicles contain glucose-6-phosphatase (18, 19) which can effectively utilize mannose-6-P as a substrate, provided the permeability barrier of the vesicles has been disrupted to allow the substrate access to the active site located on the lumenal surface (4). An exact correspondence between mannose- 6-phosphate activity and membrane permeability to EDTA has been established (4). The latency of mannose-6-phosphatase activity provides a quantitative index of microsomal integrity (4.) Few of the microsomal enzymes in the synthesis of phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol have been solubilized and/or purified, and little is known about the topography of these enzymes in the transverse or lateral planes of the endoplasmic reticulum. An asymmetric location of these biosynthetic enzymes on the cytoplasmic or lumenal surface of microsomal vesicles may provide a mechanism for regulation of the glycerolipids to be retained or secreted by the cell, and for the biogenesis of asymmetric phospholipid bilayers. In this paper, we report investigations on the localization of all seven microsomal enzymes (Scheme I) in the biosynthesis of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine, using the protease technique with mannose-6-phosphatase serving as luminal control activity. The latency of these lipid biosynthetic enzymes was also investigated, using the latency of mannose-6-phosphatase as an index of microsomal integrity.

Animals↗
PubMed1978

Two distinct mechanisms for redistribution of lymphocyte surface macromolecules. I. Relationship to cytoplasmic myosin.

A detailed kinetic analysis of the distribution of cytoplasmic myosin during the capping of various lymphocytic surface molecules revealed two distinct capping mechanisms. (a) Some cell surface molecules, including immunoglobulin, Fc receptor, and thymus leukemia antigen, all cap spontaneously in a small fraction of lymphocytes during locomotion. Cytoplasmic myosin becomes concentrated in the cytoplasm underlying these spontaneous caps. Exposure to specific antibodies causes all three of these surface molecules to cap rapidly with a concomitant redistribution of cytoplasmic myosin to the area of the cap. These antibodies also stimulate cell locomotion. (b) Other lymphocyte surface molecules, including H2 and Thy.1, do not cap spontaneously. Moreover, exposure to antibodies to these molecules causes them to cap slowly without a redistribution of cytoplasmic myosin or stimulation of cell locomotion. Exposure to concanavalin A gives a response intermediate between these two extremes. We believe that the first type of capping is active and may involve a direct link between the surface molecules and the cytoplasmic contractile apparatus. The second type of capping appears to result simply from aggregation of cross-linked molecules in the plane of the membrane.

Animals↗
PubMed1993

The beta 4 subunit cytoplasmic domain mediates the interaction of alpha 6 beta 4 integrin with the cytoskeleton of hemidesmosomes.

The alpha 6 beta 4 integrin is structurally distinct from all the other known integrins because the cytoplasmic domain of beta 4 is unusually large and contains four type III fibronectin-like modules toward its C-terminus. To examine the function of the beta 4 cytoplasmic tail, we have expressed full-length and truncated human beta 4 cDNAs in rat bladder epithelial 804G cells, which form hemidesmosome-like adhesions in vitro. The cDNA encoded wild-type beta 4 subunit associated with endogenous alpha 6 and was recruited at the cell surface within hemidesmosome-like adhesions. A recombinant form of beta 4, lacking almost the entire cytoplasmic domain associated with alpha 6, reached the cell surface but remained diffusely distributed. A beta 4 molecule lacking almost the entire extracellular portion did not associate with alpha 6 but was correctly targeted to the hemidesmosome-like adhesions. Thus, the cytoplasmic portion of beta 4 contains sequences that are required and may be sufficient for the assembly of the alpha 6 beta 4 integrin into hemidesmosomes. To localize these sequences we examined the properties of additional mutant forms of beta 4. A truncated beta 4 subunit, lacking the most C-terminal pair of type III fibronectin homology domains, was incorporated into hemidesmosome-like adhesions, but another recombinant beta 4 molecule, lacking both pairs of type III fibronectin repeats, was not. Finally a recombinant beta 4 molecule, which was created by adjoining the region of the cytoplasmic domain including all type III repeats to the transmembrane segment, was efficiently recruited in hemidesmosome-like adhesions. Taken together these results suggest that the assembly of the alpha 6 beta 4 integrin into hemidesmosomes is mediated by a 303-amino acid region of beta 4 tail that comprises the first pair of type III fibronectin repeats and the segment between the second and third repeats. These data imply a function of a specific segment of the beta 4 cytoplasmic domain in interaction with cytoskeletal components of hemidesmosomes.

Animals↗
PubMed1996

Wolbachia infection and cytoplasmic incompatibility in Drosophila species.

Forty-one stocks from 30 Drosophila species were surveyed for Wolbachia infection using PCR technology. D. sechellia and two strains of D. auraria were found to be infected and were tested for the expression of cytoplasmic incompatibility, along with D. ananassae and D. melanogaster strains, which are already known to be infected. D. ananassae and D. melanogaster show levels of incompatibility up to 25%, while D. auraria and D. sechellia exhibit levels of egg mortality approximately 60%. A dot-blot assay using the dnaA sequence as probe was developed to assess the infection levels in individual males that were used in incompatibility crosses. A positive correlation between bacterial density and cytoplasmic incompatibility was observed. The stocks examined can be clustered into at least two groups, depending on the levels of infection relative to the degree of cytoplasmic incompatibility exhibited. One group, containing D. simulans Hawaii, D. sechellia, and D. auraria, exhibits high levels of cytoplasmic incompatibility relative to levels of infection; all the other species and D. simulans Riverside exhibit significantly lower levels of cytoplasmic incompatibility relative to levels of infection. These data show that, in addition to bacterial density, bacterial and/ or host factors also affect the expression of cytoplasmic incompatibility.

Animals↗
PubMedDate not supplied

Mitochondrial transcript processing and restoration of male fertility in T-cytoplasm maize.

Cytoplasmic male sterility (CMS) systems have been useful in the production of hybrid seed in a number of crops. The Texas or T-cytoplasmic male-sterile (cms-T) system was used extensively in the 1960s to eliminate the need for hand detasseling in hybrid maize production. As a consequence of the 1970 epidemic of southern corn leaf blight, cms-T is no longer widely used commercially. However, it has been developed as a model system to study the genetic and molecular mechanisms underlying male sterility and fertility restoration. Male sterility in T-cytoplasm maize results from the action of a T-cytoplasm-specific mitochondrial gene, T-urf13. Full (or partial) fertility restoration of T-cytoplasm maize is mediated by the Rf2 nuclear restorer in combination with one of three other restorers: Rf1, Rf8, or Rf*. Rf2 encodes a protein highly similar to mitochondrial aldehyde dehydrogenases; Rf1, Rf8, and Rf* each mediate discrete T-urf13 mitochondrial transcript processing events. To test the functionality of Rf1, Rf8, or Rf*, a T-cytoplasm transformation system is under development. AFLP bulk-segregant analysis has been used to identify DNA markers closely linked to the Rf8 locus. These tools will provide a foundation for determining mechanisms of nuclear-directed mitochondrial RNA processing and fertility restoration.

Alleles↗
PubMed2006

Outbreeding selects for spiteful cytoplasmic elements.

In addition to their nuclear genome, the vast majority of eukaryotes harbour cytoplasmic genomes, e.g. in mitochondria or chloroplasts. In the majority of cases, these cytoplasmic genomes are transmitted maternally only, leading to selective pressures divergent from those that act on nuclear genes. In particular, cytoplasmic genes, which reduce the fitness of males that carry them, but have no fitness effect in females, are believed to be selectively neutral. Here, we go a step further and argue that in outbreeding populations (i.e. populations with inbreeding avoidance), 'spiteful' cytoplasmic elements that reduce the number of offspring produced by males are in fact selected for. We study this process by means of a stochastic model, analysing both the probability of spread and the impact that such a spiteful cytotype can have on population dynamics. Our results demonstrate that the probability of spread of the spiteful cytotype can be several times higher in outbreeding than in panmictic populations. Spread and fixation of the spiteful cytotype can lead to different qualitative effects on the population dynamics, including extinction, decreased or increased stable population size. We discuss our results in respect to cytoplasmically induced male infertility and cytoplasmic incompatibility.

Animals↗
PubMed1981

Chilling Susceptibility of the Blue-green Alga Anacystis nidulans: II. STIMULATION OF THE PASSIVE PERMEABILITY OF CYTOPLASMIC MEMBRANE AT CHILLING TEMPERATURES.

Potassium ions and amino acids were found to leak from the cytoplasm to the outer medium when the blue-green alga, Anacystis nidulans, was exposed to the chilling temperatures. The leakage was marked below the critical temperature regions, the midpoint values for which were around 5 and 14 C in cells grown at 28 and 38 C, respectively. These temperature regions coincided with those critical for the susceptibility of the photosynthetic activities and the carotenoid absorption spectrum previously studied (Ono TA, N Murata 1981 Plant Physiol 67: 176-181).Potassium and magnesium ions in the cell suspension medium protected the algal cells from the chilling-induced damage of the Hill reaction with 1,4-benzoquinone. The activity of the Hill reaction which had been diminished by the first chilling treatment in a low salt medium was restored by the second chilling treatment of a high salt medium. The chilling susceptibility of the Hill reaction could be attributed to the leakage of cations from the cytoplasm due to increased permeability of the cytoplasmic membrane at the chilling temperatures.A mechanism is proposed to interpret the chilling susceptibility of A. nidulans: (a) at chilling temperatures, the bilayer lipids of the cytoplasmic membrane are in the phase separation state; (b) ions and solutes having low molecular weights leak from the cytoplasm to the outer medium when the lipids of the cytoplasmic membrane are in the phase separation state; (c) decreases in the intracellular concentrations of ions and solutes degrade the physiological activities of the cells.

Journal Article↗
PubMed1982

Chilling-Susceptibility of the Blue-Green Alga Anacystis nidulans: III. LIPID PHASE OF CYTOPLASMIC MEMBRANE.

The lipid phase of cytoplasmic membrane was studied by freeze-fracture electron microscopy in the chilling-susceptible blue-green alga, Anacystis nidulans. At growth temperatures, intramembrane particles were distributed at random in the fracture faces of cytoplasmic membrane, whereas, at chilling temperatures, the fracture faces were composed of particle-free and particle-containing regions. These findings indicate that lipids of the cytoplasmic membrane were in the liquid-crystalline state at the growth temperatures and in the phase-separation state at the chilling temperatures. Temperatures for the onset of phase separation were 5 and 16 degrees C in cells grown at 28 and 38 degrees C, respectively.In comparison, another blue-green alga, Anabaena variabilis, which is not susceptible to chilling, was also examined by the freeze-fracture electron microscopy. The intramembrane particles were distributed at random in the fracture faces of cytoplasmic membrane at the growth, as well as at the chilling, temperatures.The results in this and previous studies suggest that the chilling susceptibility of A. nidulans is a result of irreversible leakage of ions from the cytoplasm when the lipids of cytoplasmic membrane are in the phase-separation state at low temperatures.

Journal Article↗
PubMed1982

Regulation of Cytoplasmic and Vacuolar pH in Maize Root Tips under Different Experimental Conditions.

(31)P-Nuclear magnetic resonance spectra of perfused maize (Zea mays L., hybrid WW x Br 38) root tips, obtained at 10-minute intervals over 12 hours or longer, indicate that no cytoplasmic or vacuolar pH changes occur in these cells in the presence of 25 millimolar K(2)SO(4), which induces extrusion of 4 to 5 microequivalents H(+) per gram per hour. In contrast, hypoxia causes cytoplasmic acidification (0.3-0.6 pH unit) without a detectable change in vacuolar pH. The cytoplasm quickly returns to its original pH on reoxygenation. Dilute NH(4)OH increases the vacuolar pH more than it does the cytoplasmic pH; after NH(4)OH is removed, the vacuole recovers its original pH more slowly than does the cytoplasm. The results indicate that regulation of cytoplasmic pH and that of vacuolar pH in plant cells are separate processes.

Journal Article↗
PubMed1986

Cytoplasmic pH Regulation in Acer pseudoplatanus Cells: I. A P NMR Description of Acid-Load Effects.

Modifications of cytoplasmic pH in Acer pseudoplatanus L. cells cultivated in suspension have been induced by acid-loads and studied by using (31)P nuclear magnetic resonance spectroscopy. The initial drop of cytoplasmic pH, observed in the first minutes of exposure to weak lipophilic acids, was followed by a slow recovery to reach a plateau phase with a pH value lower than the initial one. Conversely, removal of the acid led to a sharp increase of cytoplasmic pH with in most cases an overshoot toward more alkaline values than the initial one and a subsequent decrease to more acidic values. This shows that A. pseudoplatanus cells powerfully regulate their cytoplasmic pH both on the acid side of their normal pH, during the acid-load, and on the alkaline side, after removal of acid. Similar results were obtained with different types of acid-loads, i.e. treatments with propionic or benzoic acid or bubbling with CO(2)-enriched air. This indicates that the occurrence of pH regulation does not depend upon the method used to acid-load the cells. The time courses of cytoplasmic pH observed for A. pseudoplatanus and also Catharanthus roseus cells are similar to those recorded for animal cells but different from those described for other plant materials for which no recovery phase was observed. This can be explained by different balances between the initial rate of proton influx brought in by the acids, and the capacity of proton consumption by the regulatory mechanisms. The existence of the recovery phase offers a unique possibility to study the regulation of the cytoplasmic pH of plant cells, as it has been done in animal systems.

Journal Article↗