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PubMed2004

Val133 and Cys137 in transmembrane segment 2 are close to Arg935 and Gly939 in transmembrane segment 11 of human P-glycoprotein.

P-glycoprotein (P-gp; ABCB1) transports a wide variety of structurally diverse compounds out of the cell. The protein has two homologous halves joined by a linker region. Each half consists of a transmembrane (TM) domain with six TM segments and a nucleotide-binding domain. The drug substrate-binding pocket is at the interface between the TM segments in each half of the protein. Preliminary studies suggested that the arrangement of the two halves of P-gp shows rotational symmetry (i.e. "head-to-tail" arrangement). Here, we tested this model by determining whether the cytoplasmic ends of TM2 and TM3 in the N-terminal half are in close contact with TM11 in the C-terminal half. Mutants containing a pair of cysteines in TM2/TM11 or TM3/TM11 were subjected to oxidative cross-linking with copper phenanthroline. Two of the 110 TM2/TM11 mutants, V133C(TM2)/G939C(TM11) and C137C(TM2)/A935C (TM11), were cross-linked at 4 degrees C, when thermal motion is reduced. Cross-linking was specific since no cross-linked product was detected in the 100 double Cys TM3/TM11 mutants. Vanadate trapping of nucleotide or the presence of some drug substrates inhibited cross-linking of mutants V133C(TM2)/G939C(TM11) and C137C(TM2)/A935C(TM11). Cross-linking of TM2 and TM11 also blocked drug-stimulated ATPase activity. The close proximity of TM2/TM11 and TM5/TM8 (Loo, T. W., Bartlett, M. C., and Clarke, D. M. (2004) J. Biol. Chem. 279, 7692-7697) indicates that these regions between the two halves must enclose the drug-binding pocket at the cytoplasmic side of P-gp. They may form the "hinges" required for conformational changes during the transport cycle.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2005

Exploring the binding domain of EmrE, the smallest multidrug transporter.

EmrE is a small multidrug transporter in Escherichia coli that extrudes various positively charged drugs across the plasma membrane in exchange with protons, thereby rendering cells resistant to these compounds. Biochemical experiments indicate that the basic functional unit of EmrE is a dimer where the common binding site for protons and substrate is formed by the interaction of an essential charged residue (Glu14) from both EmrE monomers. Previous studies implied that other residues in the vicinity of Glu14 are part of the binding domain. Alkylation of Cys replacements in the same transmembrane domain inhibits the activity of the protein and this inhibition is fully prevented by substrates of EmrE. To monitor directly the reaction we tested also the extent of modification using fluorescein-5-maleimide. While most residues are not accessible or only partially accessible, four, Y4C, I5C, L7C, and A10C, were modified at least 80%. Furthermore, preincubation with tetraphenylphosphonium reduces the reaction of two of these residues by up to 80%. To study other essential residues we generated functional hetero-oligomers and challenged them with various methane thiosulfonates. Taken together the findings imply the existence of a binding cavity accessible to alkylating reagents where at least three residues from TM1, Tyr40 from TM2, and Trp63 in TM3 are involved in substrate binding.

Antiporters↗
PubMed2006

The residues determining differences in ion affinities among the alternative splice variants F, A, and B of the mammalian renal Na-K-Cl cotransporter (NKCC2).

Three alternatively spliced variants of the renal Na-K-Cl cotransporter (NKCC2) are found in distinct regions of the thick ascending limb of the mammalian kidney; these variants mediate Na(+)K(+)2Cl(-) transport with different ion affinities. Here, we examine the specific residues involved in the variant-specific affinity differences, utilizing a mutagenic approach to change the NKCC2B variant into the A or F variant, with functional expression in Xenopus oocytes. The splice region contains the second transmembrane domain (TM2) and the putative intracellular loop (ICL1) connecting TM2 and TM3. It is found that the B variant is functionally changed to the F variant by replacement of six residues, half of the effect brought about by three TM2 residues and half by three ICL1 residues. The involvement of the ICL1 residues strongly suggests that this region of ICL1 may actually be part of a membrane-embedded domain. Changing six residues is also sufficient to bring about the smaller functional change from the B to the A variant; three residues in TM2 appear to be primarily responsible, two of which correspond to residues involved in the B-to-F changes. A B-variant mutation reported in a mild case of Bartter disease was found to render the cotransporter inactive. These results identify the combination of amino acid variations responsible for the differences among the three splice variants of NKCC2, and they support a model in which a reentrant loop following TM2 contributes to the chloride binding and translocation domains.

Amino Acid Substitution↗
PubMed2005

Noninvasive thermometry using hyperfine-shifted MR signals from paramagnetic lanthanide complexes.

MR thermometry techniques based on the strong water 1H signal provide high spatial and temporal resolution and have shown promise for applications such as laser surgery and RF ablation. However, these techniques have low temperature sensitivity for hyperthermia applications and are greatly influenced by local motion and susceptibility variations. 1H NMR signals from paramagnetic lanthanide complexes of Pr3+, Yb3+ and Tm3+ show up to 300-fold stronger temperature dependence compared to the water 1H signal. In addition, 1H chemical shifts of many of these complexes are insensitive to other factors such as the concentration of the paramagnetic complex, pH, [Ca2+], and the presence of plasma macro-molecules and ions. Applications of lanthanide complexes for temperature measurement in intact animals and the feasibility of mapping temperatures in phantoms have been demonstrated. Among all the lanthanide complexes examined so far, thulium 1, 4, 7, 10-tetramethyl-1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetate (TmDOTMA-) appears to be the most attractive for in vivo MR thermometry. The 1H signal from the methyl groups on this complex is relatively intense because of 12 equivalent protons and provides high temperature sensitivity because of the large paramagnetic shifts induced by thulium. The possibility of imaging TmDOTMA2--in intact animals at physiologically safe concentrations has recently been demonstrated. Overall, MR thermometry methods based on hyperfine-shifted MR signals from paramagnetic lanthanide complexes appear promising for animal applications, but further studies relating to acceptable dose and signal-to-noise ratio are necessary before clinical use.

Animals↗
PubMed2004

Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy.

Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

Animals↗
PubMed2003

Single-channel SCAM identifies pore-lining residues in the first extracellular loop and first transmembrane domains of Cx46 hemichannels.

Gap junction (GJ) channels provide an important pathway for direct intercellular transmission of signaling molecules. Previously we showed that fixed negative charges in the first extracellular loop domain (E1) strongly influence charge selectivity, conductance, and rectification of channels and hemichannels formed of Cx46. Here, using excised patches containing Cx46 hemichannels, we applied the substituted cysteine accessibility method (SCAM) at the single channel level to residues in E1 to determine if they are pore-lining. We demonstrate residues D51, G46, and E43 at the amino end of E1 are accessible to modification in open hemichannels to positively and negatively charged methanethiosulfonate (MTS) reagents added to cytoplasmic or extracellular sides. Positional effects of modification along the length of the pore and opposing effects of oppositely charged modifying reagents on hemichannel conductance and rectification are consistent with placement in the channel pore and indicate a dominant electrostatic influence of the side chains of accessible residues on ion fluxes. Hemichannels modified by MTS-EA+, MTS-ET+, or MTS-ES- were refractory to further modification and effects of substitutions with positively charged residues that electrostatically mimicked those caused by modification with the positively charged MTS reagents were similar, indicating all six subunits were likely modified. The large reductions in conductance caused by MTS-ET+ were visible as stepwise reductions in single-channel current, indicative of reactions occurring at individual subunits. Extension of single-channel SCAM using MTS-ET+ into the first transmembrane domain, TM1, revealed continued accessibility at the extracellular end at A39 and L35. The topologically complementary region in TM3 showed no evidence of reactivity. Structural models show GJ channels in the extracellular gap to have continuous inner and outer walls of protein. If representative of open channels and hemichannels, these data indicate E1 as constituting a significant portion of this inner, pore-forming wall, and TM1 contributing as pore-lining in the extracellular portion of transmembrane span.

Animals↗
PubMed1996

Sequence requirements for membrane assembly of polytopic membrane proteins: molecular dissection of the membrane insertion process and topogenesis of the human MDR3 P-glycoprotein.

The biogenesis of membrane proteins with a single transmembrane (TM) segment is well understood. However, understanding the biogenesis and membrane assembly of membrane proteins with multiple TM segments is still incomplete because of the complexity and diversity of polytopic membrane proteins. In an attempt to investigate further the biogenesis of polytopic membrane proteins, I used the human MDR3 P-glycoprotein (Pgp) as a model polytopic membrane protein and expressed it in a coupled cell-free translation/translocation system. I showed that the topogenesis of the C-terminal half MDR3 Pgp molecule is different from that of the N-terminal half. This observation is similar to that of the human MDR1 Pgp. The membrane insertion properties of the TM1 and TM2 in the N-terminal half molecule are different. The proper membrane anchorage of both TM1 and TM2 of the MDR3 Pgp is affected by their C-terminal amino acid sequences, whereas only the membrane insertion of the TM1 is dependent on the N-terminal amino acid sequences. The efficient membrane insertion of TM3 and TM5 of MDR3 Pgp, on the other hand, requires the presence of the putative TM4 and TM6, respectively. The TM8 in the C-terminal half does not contain an efficient stop-transfer activity. These observations suggest that the membrane insertion of putative TM segments in the human MDR3 Pgp does not simply follow the prevailing sequential event of the membrane insertion by signal-anchor and stop-transfer sequences. These results, together with my previous findings, suggest that different isoforms of Pgp can be used in comparison as a model system to understand the molecular mechanism of topogenesis of polytopic membrane proteins.

ATP Binding Cassette Transporter, Subfamily B↗
PubMed2004

Making sense of olfaction through predictions of the 3-D structure and function of olfactory receptors.

We used the MembStruk first principles computational technique to predict the three-dimensional (3-D) structure of six mouse olfactory receptors (S6, S18, S19, S25, S46 and S50) for which experimental odorant recognition profiles are available for a set of 24 odorants (4-9 carbons aliphatic alcohols, acids, bromo-acids and diacids). We used the HierDock method to scan each predicted OR structure for potential odorant binding site(s) and to calculate binding energies of each odorant in these binding sites. The calculated binding affinity profiles are in good agreement with experimental activation profiles, validating the predicted 3-D structures and the predicted binding sites. For each of the six ORs, the binding site is located between trans-membrane domains (TMs) 3-6, with contributions from extracellular loops 2 and 3. In particular, we find six residue positions in TM3 and TM6 to be consistently involved in the binding modes of the odorants. Indeed, the differences in the experimental recognition profiles can be explained on the basis of these critical residues alone. These predictions are also consistent with mutation data on ligand binding for catecholamine receptors and sequence hypervariability studies for ORs. Based on this analysis, we defined amino acid patterns associated with the recognition of short aliphatic alcohols and mono-acids. Using these two sequence fingerprints to probe the alignment of 869 OR sequences from the mouse genome, we identified 34 OR sequences matching the fingerprint for aliphatic mono-acids and 36 corresponding to the recognition pattern for aliphatic alcohols. We suggest that these two sets of ORs might function as basic arrays for uniquely recognizing aliphatic alcohols and acids. We screened a library of 89 additional molecules against the six ORs and found that this set of ORs is likely to respond to aldehydes and esters with longer carbon chains than their currently known agonists. We also find that compounds associated with the flavor in foods are often among the best calculated binding affinities. This suggests that physiologic ligands for these ORs may be found among aldehydes and esters associated with flavor.

Acids↗
PubMed2003

Non-syndromic vestibular disorder with otoconial agenesis in tilted/mergulhador mice caused by mutations in otopetrin 1.

Otoconia are biominerals within the utricle and saccule of the inner ear that are critical for the perception of gravity and linear acceleration. The classical mouse mutant tilted (tlt) and a new allele, mergulhador (mlh), are recessive mutations that affect balance by impairing otoconial morphogenesis without causing collateral deafness. The mechanisms governing otoconial biosynthesis are not known. Here we show that tlt and mlh are mutant alleles of a novel gene (Otopetrin 1, Otop1), encoding a multi-transmembrane domain protein that is expressed in the macula of the developing otocyst. Both mutants carry single point mutations leading to non-conservative amino acid substitutions that affect two putative transmembrane (TM) domains (tlt, Ala(151)-->Glu in TM3; mlh, Leu(408)-->Gln in TM8). Otop1 and Otop1-like paralogues, Otop2 and Otop3, define a new gene family with homology to the C. elegans and D. melanoganster DUF270 genes.

Alleles↗
PubMed2003

Antiquity and evolution of the MADS-box gene family controlling flower development in plants.

MADS-box genes in plants control various aspects of development and reproductive processes including flower formation. To obtain some insight into the roles of these genes in morphological evolution, we investigated the origin and diversification of floral MADS-box genes by conducting molecular evolutionary genetics analyses. Our results suggest that the most recent common ancestor of today's floral MADS-box genes evolved roughly 650 MYA, much earlier than the Cambrian explosion. They also suggest that the functional classes T (SVP), B (and Bs), C, F (AGL20 or TM3), A, and G (AGL6) of floral MADS-box genes diverged sequentially in this order from the class E gene lineage. The divergence between the class G and E genes apparently occurred around the time of the angiosperm/gymnosperm split. Furthermore, the ancestors of three classes of genes (class T genes, class B/Bs genes, and the common ancestor of the other classes of genes) might have existed at the time of the Cambrian explosion. We also conducted a phylogenetic analysis of MADS-domain sequences from various species of plants and animals and presented a hypothetical scenario of the evolution of MADS-box genes in plants and animals, taking into account paleontological information. Our study supports the idea that there are two main evolutionary lineages (type I and type II) of MADS-box genes in plants and animals.

Animals↗
PubMed1996

Induction of somatic mutation and recombination by four inhibitors of eukaryotic topoisomerases assayed in the wing spot test of Drosophila melanogaster.

Four inhibitors of eukaryotic topoisomerases were investigated for genotoxic effects in the wing spot test of Drosophila melanogaster. As a somatic mutation and recombination test (SMART) this assay assesses mitotic recombination and mutational events of various kinds. We studied camptothecin as a topoisomerase I inhibitor, as well as ellipticine as an intercalating inhibitor and teniposide and etoposide as two non-intercalating inhibitors of topoisomerase II. Wing spots were induced in flies trans-heterozygous for the recessive wing cell markers multiple wing hairs (mwh) and flare (flr3) as well as in flies heterozygous for mwh and the multiply inverted TM3 balancer chromosome. All four compounds proved significantly genotoxic in this test. The spot induction frequencies formally standardized to the millimolar unit of exposure dose decreased in the order camptothecin > teniposide > ellipticine greater, similar etoposide in the mwh/flr3 inversion-free genotype. In the mwh/TM3 genotype, in which mitotic crossing over is suppressed because of the inversion-heterozygosity, the observed spot frequencies were considerably reduced, but to different extents. In this genotype, spot induction by ellipticine was not statistically significant, and it was determined that >99% of the spots are due to mitotic recombination in mwh/flr3 flies. For the other compounds, spot induction in the inversion-heterozygous genotype was significant. The relative contribution of recombination to total spot induction in the inversion-free genotype was 88% for camptothecin. It was significantly lower for the two epipodophyllotoxins teniposide (71%) and etoposide (59%). Only suggestions can be proffered at present as to how these proportions could be related to the primary damage produced by the respective compounds on the chromosomes.

Animals↗
PubMed1997

Enormously fast RNA hydrolysis by lanthanide(III) ions under physiological conditions: eminent candidates for novel tools of biotechnology.

Lanthanide(III) ions have shown enormous catalyses for the hydrolysis of the phosphodiester linkages in RNA, indicating their high potential for versatile applications to biotechnology and molecular biology. The activity monotonically increases with increasing atomic number in the lanthanide series, the last three ions (Tm3+, Yb3+, and Lu3+) being the most active. Non-lanthanide metal ions are virtually inactive. The pseudo first-order rate constant for the hydrolysis of adenylyl(3'-5')adenosine (ApA) by LuCl3 (5 mmol x dm(-3)) at pH 7.2 and 30 degrees C is 1.9 x 10(-1) min(-1) (the half-life is only 3.6 min), corresponding to 10(8)-fold acceleration. The product is an equimolar mixture of adenosine and its 2'- or 3'-monophosphate without any byproducts. The 2',3'-cyclic monophosphate of adenosine is not accumulated much in the reaction mixture. Lanthanide ions also efficiently hydrolyze oligoribonucleotides without a specific base-preference. In ApA hydrolysis by NdCl3 and GdCl3, the dependence of the hydrolysis rate on either the pH or concentration of the metal salt coincides fairly well with the corresponding profile of the equilibrium concentration of the bimetallic hydroxo-cluster [M2(OH)2]4+ (M=metal ion). Both the formation of the pentacoordinated intermediate and its decomposition are greatly promoted by lanthanide ions. A catalytic mechanism in which two metal ions (or their coordination water) in these tetracationic hydroxo-clusters show acid/base cooperation is proposed.

Adenosine Monophosphate↗
PubMed1979

Effect of butyrate on glycolipid metabolism of two cell types of rat ascites hepatomas with different ganglioside biosynthesis.

The effect of butyrate on glycolipid metabolism and morphological differentiation in the cell culture system of rat ascites hepatomas, AH 7974 of island-forming type and AH 7974F of free type, was studied. Both cell lines adhered to the substratum in the presence of 1 mM butyrate. In the case of AH 7974, the addition of butyrate induced a distinct morphological change but the other cell line showed no such conspicuous change. Butyrate-treated AH 7974 cells showed a 2 to 3-fold elevation of CMP-N-acetylneuraminic acid: lactosylceramide sialyltransferase activity to form N-acetylneuraminylgalactosylglucosylceramide (GM3). On the other hand, no enzyme activity could be detected in AH 7974F cells. Four glycosyltransferase activities involved in glycolipid synthesis, including sialyltransferase in AH 7974F cells, were reduced by butyrate. From these observations we concluded that sialyltransferase to form GM3, or TM3 itself, is prerequisite for the morphological alteration induced by butyrate.

Animals↗
PubMed1986

Acid phosphatases in germinal and somatic cells of the testes.

Four forms of acid phosphatase have been found in the testicular tissue of many mammalian species, but their exact cellular site has remained obscure. In this work, acid phosphatases have been studied in different reproductive organs of the male rat, in somatic cell lines derived by cloning from both rat and mouse testes, in primary cultures of rat Sertoli cells, and in isolated spermatogenic cells of the mouse. Among the reproductive organs, preputial glands show the highest specific activities with p-nitrophenyl phosphate as substrate, followed by the testicular tissue and the different regions of the epididymis. By contrast to that in other tissues, testicular activity with p-nitrophenyl phosphate is not influenced by tartrate and is activated markedly by cobalt (Co2+). Among the somatic cell lines, the highest hydrolysis rates are obtained with naphthyl substrates in the epithelial (TR-1) and myoid (TR-M) cell lines and marginally lower rates in the Leydig (TM3) and Sertoli (TM4) cell lines. With thymolphthalein phosphate, the latter two cell lines show very low activity. These activities are not influenced by different hormones and growth factors in the culture medium. The most marked Co2+-activated reaction with p-nitrophenyl phosphate is found in advanced stages of germinal cells and residual bodies. Primary cultures of Sertoli cells, prepared from rats 10 to 30 days of age, show a slight decrease in acid phosphatase levels; however, the activities are not influenced markedly by addition of follicle-stimulating hormone (FSH) and/or testosterone to the culture medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid Phosphatase↗
PubMed2001

Abnormal thiol reactivity of tropomyosin in essential hypertension and its association with abnormal sodium-lithium countertransport kinetics.

OBJECTIVES: To identify a thiol protein that is abnormal in a subgroup of essential hypertensive (EHT) patients who have a strong family history of hypertension and cardiovascular disease and have a low Km of erythrocyte Na/Li countertransport (CT). METHODS: To detect biotin maleimide labelling of a key thiol protein to investigate its reaction with N-ethylmaleimide (NEM) in normal and EHT erythrocytes. RESULTS: The thiol protein of 33 kDa apparent molecular weight (p33) identified by the loss of labelling with biotin maleimide was identified as tropomyosin due to its retarded running in 6 mol/l urea gels and immunoblotting. The NEM reaction with p33 detected by loss of subsequent biotin maleimide labelling is biphasic in normal control erythrocytes with the rate in the first 30 s double that after 30 s. In EHT erythrocytes NEM reaction (1) after 30 s is faster than normal and (2) in the first 30 s causes a paradoxical increase in apparent biotin maleimide labelling. In normal control erythrocytes, the loss of biotin maleimide labelling with NEM reaction or the faster phenylmaleimide reaction follows the same time course as the decrease in Km of Na/Li CT. CONCLUSIONS: NEM reaction with p33 requires two thiols. Only the cytoskeletal form of tropomyosin from the TM3 gene has more than one thiol group and agrees with SDS-PAGE mobility. Tropomyosin is a strong candidate to explain the familial abnormality in EHT with abnormal Na/ Li CT and it could explain many of the characteristics of this disease.

Biotin↗
PubMed2003

Vanilloid receptor TRPV1 is not activated by vanilloids applied intracellularly.

The vanilloid receptor TRPV1 is a ligand-gated cation channel that can be activated by capsaicin, acids and noxious heat. For vanilloids, a stretch of approximately 8 amino acids in the vicinity of the TM3 region on the cytosolic side of TRPV1 and R114 and E761 in the N- and C-cytosolic tails, respectively, has been shown to be critical for capsaicin binding and channel activation. Here, we report that intracellular application of vanilloids is insufficient for activating TRPV1 channels in HEK293T cells. Pipette solution (ICS) for recording membrane currents was supplemented with 50 microM capsaicin (n=14) or 1 microM resiniferatoxin (RTX) (n=39) and the responses induced by extracellular capsaicin (1 microM) or RTX (100 nM) were recorded at intervals >50% of that needed for diffusion of Lucifer yellow from the pipette to reach maximum fluorescence (n=7). We found that all cells with expressed TRPV1 exhibited a similar sensitivity to vanilloids irrespective of whether the membrane currents were recorded with electrodes filled with ICS containing capsaicin or RTX or only with control ICS. We suggest that, in addition to intracellularly located agonist recognition sites of TRPV1, there is at least one resides on the extracellular side, which needs to be occupied to activate the channel.

Animals↗
PubMed1986

Development-dependent replication of minute virus of mice in differentiated mouse testicular cell lines.

The replication of the autonomous parvovirus, minute virus of mice (MVM), requires mitotically active cells and depends on certain factors expressed by cells of particular differentiated phenotype. As an approach to the understanding of these helper functions, we studied the interaction of the fibrotropic [MVM(p)] and the lymphotropic [MVM(i)] strains of MVM with two differentiated cell lines from mouse testicular epithelial origins. The relative support given to viral expression by these cell lines varied extensively. Cells from Sertoli origin (TM4) were permissive to MVM(p) but were mostly restrictive to MVM(i). The other cell line, of Leydig cell origin (TM3), was highly restrictive to both viral strains, but the blocks to their growth in these cells were localized at different stages of their growth cycle, suggesting that the replication of MVM in these cells requires tissue-specific helper functions during at least two stages of viral replication.

Animals↗
PubMed2000

Intrinsic bistability of luminescence and stimulated emission in Yb- and Tm-doped glass

Intrinsic bistability and hysteresis have been observed in the photoluminescence of bulk glass doped with Yb3+ and Tm3+ impurities at room temperature. Multiple instabilities also occur in channel waveguide lasers in this material. Consequently, local field effects exhibit more degrees of freedom than expected and have important implications for compact optical amplifiers and oscillators.

Journal Article↗