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Multiple isoforms and an unusual cathodic isoform of creatine kinase from channel catfish (Ictalurus punctatus).

In vertebrates, the creatine kinase (CK) family consists of two cytosolic and two mitochondrial isoforms. The two cytosolic isoforms are the muscle type (M-CK) and the brain type (B-CK). Here we report multiple CK isoenzymes in the diploid channel catfish (Ictalurus punctatus) with one unusual cathodic isoform that was previously found only in pathological situations in human. The cathodic CK isoform existed only in the channel catfish stomach, ovary, and spleen, but not in any other species analyzed such as tilapia, smallmouth bass, chicken, or rat. Two genes encode the multiple forms of the channel catfish M-CK cDNAs. M-CK1 has three alleles, M-CK1.1, M-CK1.2, and M-CK1.3, while M-CK2 has just one allele as determined by analysis of 17 cDNA clones and by allele-specific PCR. M-CK1 encodes a protein of 381 amino acids and the M-CK2 cDNA encodes a protein of 380 amino acids. The two cDNAs shared an 86% identity and both have the nine diagnostic boxes for cytosolic CKs and thus are of cytosolic origin. The M-CK1 gene was isolated, sequenced, and characterized and its promoter should be useful for transgenic research for muscle-specific expression.

Amino Acid Sequence↗

Rapid plasmenylethanolamine-selective fusion of membrane bilayers catalyzed by an isoform of glyceraldehyde-3-phosphate dehydrogenase: discrimination between glycolytic and fusogenic roles of individual isoforms.

Recently we demonstrated that the unique stereoelectronic relationships inherent in the structure of plasmenylethanolamine facilitate membrane fusion, and we postulated the existence of a membrane fusion protein which could exploit the propensity of plasmenylethanolamine molecular species to adapt an inverted hexagonal phase [Glaser & Gross (1994) Biochemistry 33, 5805-5812]. We now report a cryptic membrane fusion activity in rabbit brain cytosol, which requires separation from an endogenous inhibitor to express its activity, and demonstrate that vesicle fusion catalyzed by this protein is highly selective for membrane vesicles containing plasmenylethanolamine. The cytosolic protein catalyzing membrane fusion activity was purified to apparent homogeneity by sequential column chromatographies, revealing a single 38-kDa protein band after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining. Automated Edman degradation demonstrated that the purified protein is an isoform of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which was confirmed by Western blot analysis utilizing polyclonal antibodies and by solution-state inactivation of membrane fusion activity by a monoclonal antibody directed against GAPDH. Both GTP-affinity and Mono Q chromatographies resolved GAPDH isoforms that catalyzed dehydrogenase activity from the GAPDH isoform that catalyzed membrane fusion activity. The purified fusion protein was calcium-independent, resistant to treatment with N-ethylmaleimide, and possessed an obligatory requirement for plasmenylethanolamine and cholesterol. High-resolution stopped-flow kinetic analysis of plasmenylethanolamine-facilitated membrane fusion demonstrated that one tetramer of the GAPDH isoform catalyzed one fusion event between two vesicles containing plasmenylethanolamine every millisecond (on average). Collectively, these results constitute the first description of a protein which can catalyze the fusion of vesicles at a rate which satisfies the mathematical constraints imposed by the observed rates of fusion of synaptic vesicles with the presynaptic membrane in vivo.

Amino Acid Sequence↗

Characterization of recombinant long-chain rat acyl-CoA synthetase isoforms 3 and 6: identification of a novel variant of isoform 6.

The metabolism of long-chain fatty acids in brain and their incorporation into signaling molecules such as diacylglycerol and LPA and into structural components of membranes, including myelin, requires activation by long-chain acyl-CoA synthetase (ACSL). Because ACSL3 and ACSL6 are the predominant ACSL isoforms in brain, we cloned and characterized these isoforms from rat brain and identified a novel ACSL6 clone (ACSL6_v2). ACSL6_v2 and the previously reported ACSL6_v1 represent splice variants that include exon 13 or 14, respectively. Homologue sequences of both of these variants are present in the human and mouse databases. ACSL3, ACSL6_v1, and ACSL6_v2 with Flag-epitopes at the C-termini were expressed in Escherichia coli and purified on Flag-affinity columns. The three recombinant proteins were characterized. Compared to ACSL4, another brain isoform, ACSL3, ACSL6_v1, and ACSL6_v2 showed similarities in kinetic values for CoA, palmitate, and arachidonate, but their apparent Km values for oleate were 4- to 6-fold lower than for ACSL4. In a direct competition assay with palmitate, all the polyunsaturated fatty acids tested were strong competitors only for ACSL4 with IC50 values of 0.5 to 5 microM. DHA was also strongly preferred by ACSL6_v2. The apparent Km value for ATP of ACSL6_v1 was 8-fold higher than that of ACSL6_v2. ACSL3 and the two variants of ACSL6 were more resistant than ACSL4 to heat inactivation. Despite the high amino acid identity between ACSL3 and ACSL4, rosiglitazone inhibited only ACSL4. Triacsin C, an inhibitor of ACSL1 and ACSL4, also inhibited ACSL3, but did not inhibit the ACSL6 variants. These data further document important differences in the closely related ACSL isoforms and show that amino acid changes near the consensus nucleotide binding site alter function in the two splice variants of ACSL6.

Alternative Splicing↗

Identification and functional expression of four isoforms of ATPase II, the putative aminophospholipid translocase. Effect of isoform variation on the ATPase activity and phospholipid specificity.

ATPase II, a vanadate-sensitive and phosphatidylserine-dependent Mg(2+)-ATPase, is a member of a subfamily of P-type ATPase and is presumably responsible for aminophospholipid translocation activity in eukaryotic cells. The aminophospholipid translocation activity plays an important physiological role in the maintenance of membrane phospholipid asymmetry that is observed in the plasma membrane as well as the membranes of certain cellular organelles. While the preparations of ATPase II from different sources share common fundamental properties, such as substrate specificity, inhibitor spectrum, and phospholipid dependence, they are divergent in several characteristics. These include specific ATPase activity and phospholipid selectivity. We report here the identification of four isoforms of ATPase II in bovine brain. These isoforms are formed by a combination of two major variations in their primary sequences and show that the structural variation of these isoforms has functional significance in both ATPase activity and phosholipid selectivity. Furthermore, studies with the phosphoenzyme intermediate of ATPase II and its recombinant isoforms revealed that phosphatidylserine is essential for the dephosphorylation of the intermediate. Without phosphatidylserine, ATPase II would be accumulated as phosphoenzyme in the presence of ATP, resulting in the interruption of its catalytic cycle.

Amino Acid Sequence↗

Functional differences between Ultrabithorax protein isoforms in Drosophila melanogaster: evidence from elimination, substitution and ectopic expression of specific isoforms.

The homeotic selector gene Ultrabithorax (Ubx) specifies regional identities in multiple tissues within the thorax and abdomen of Drosophila melanogaster. Ubx encodes a family of six developmentally specific homeodomain protein isoforms translated from alternatively spliced mRNAs. The mutant allele Ubx195 contains a stop codon in exon mII, one of three differential elements, and consequently produces functional UBX protein only from mRNAs of type IVa and IVb, which are expressed mainly in the central nervous system. Although it retains activity for other processes, Ubx195 behaves like a null allele with respect to development of the peripheral nervous system, indicating that UBX-IVa and IVb alone do not contribute detectable Ubx function for this tissue. The mutant allele UbxMX17 contains an inversion of exon mII. We find that this allele only produces mRNAs of type IVa, but the expression pattern of the resulting UBX-IVa protein is indistinguishable from that of total UBX protein expression in wild-type embryos. The phenotype of homozygous UbxMX17 embryos indicates that UBX-IVa cannot substitute functionally for other isoforms to promote normal development of the peripheral nervous system. This functional limitation is confirmed by a detailed analysis of the peripheral nervous system in embryos that express specific UBX isoforms ectopically under control of a heat shock promoter. Additional observations suggest that UBX isoforms also differ in their ability to function in other tissues.

Alleles↗

Multiple isoform recovery (MIR)-PCR: a simple method for the isolation of related mRNA isoforms.

We present a rapid and efficient method for the detection of related transcripts with different expression levels. This approach combines the rapid amplification of cDNA ends (RACE) method with a cDNA subtractive technique. The strategy is based on successive subtractions of prevalent isoforms resulting in enrichment of less expressed transcripts. For each subtraction, a biotinylated primer specific for the prevalent isoform is hybridized on the total cDNA and the hybrid is retained on a streptavidin affinity column. The unbound cDNA serves as a template for subsequent isoform identification. To illustrate its application we describe the isolation of three new actin cDNA isoforms in the freshwater planarian Dugesia (S) polychroa.

Actins↗

Distribution of the intracellular Ca(2+)-ATPase isoform 2b in pig brain subcellular fractions and cross-reaction with a monoclonal antibody raised against the enzyme isoform.

The presence and distribution of sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) isoform 2b in microsomes and other subcellular fractions isolated from pig brain has been demonstrated by the combined use of a specific antibody raised against the SERCA2b isoform and ATP phosphorylation experiments. All subcellular fractions show an approximately 110 kDa phosphorylated protein, the band intensity being stronger in microsomes. Preliminary treatment of the samples with trypsin generates two phosphorylated fragments of about 57 and 33 kDa in the presence of Ca(2+). The observed fragments are typical trypsinized products of the SERCA2b isoform. The monoclonal antibody Y/1F4 raised against the sarcoplasmic reticulum Ca(2+)-ATPase (isoform 1) binds to the 110 kDa band in membranes isolated from brain. The binding was stronger in microsomes than in other fractions. Furthermore, this antibody also recognizes a clear band at around 115 kDa. This band is always stronger in plasma membrane than in synaptosomes or microsomes and is unaffected by trypsin. Phosphorylation studies in the absence of Ca(2+) suggest that the 115 kDa protein is not a Ca(2+)-ATPase.

Animals↗

Chimeric rat Na,K-ATPase alpha 1/alpha 3* isoforms. Analysis of the structural basis for differences in Na+ requirements in the alpha 1 and alpha 3* isoforms.

Na,K-ATPase molecules containing the alpha 1, alpha 2*, and alpha 3* isoforms expressed in HeLa cells exhibit a two- to threefold difference in their K0.5 for Na+ (alpha 1 = alpha 2* < alpha 3*). To investigate the structural basis for this difference, chimeric alpha 1/alpha 3* isoform cDNAs were constructed and expressed in HeLa cells. Na,K-ATPase containing each alpha isoform chimera was analyzed for its Na+ dependence properties. Results of these experiments do not reveal a region in the alpha 1 or alpha 3* isoform that is clearly responsible for the apparent affinity for Na+. It is possible that molecular interactions involving amino acids that span virtually the entire Na,K-ATPase molecule contribute to the determination of this parameter.

Amino Acid Sequence↗

Expression of multiple Na+,K+-adenosine triphosphatase isoform genes in human hematopoietic cells. Behavior of the novel A3 isoform during induced maturation of HL60 cells.

Multiple isoenzymes of the Na+,K+-ATPase (alpha, alpha+, and alpha 3) have been identified by molecular cloning (Shull, G. E., J. Greeb, and J. B. Lingrel. 1986. Biochemistry. 25:8125-8132; and Schneider, J. W., R. W. Mercer, and E. J. Benz, Jr. 1987. Clin. Res. 35:585A. [Abstr.]). At least one of these, the alpha 3 chain, represents a novel form for which protein products and enzymatic activities are just beginning to be defined in rodents. We have recently demonstrated that expression of alpha 3 is largely confined to neuromuscular tissues of fetal and adult rats (Schneider, J. W., R. W. Mercer, M. Gilmore-Hebert, M. F. Utset, C. Lai, A. Greene, and E. J. Benz, Jr. 1988. Proc. Natl. Acad. Sci. USA. 85:284-288). We now report that certain human leukemia cell lines including HL60, HEL, and Molt 4 express mRNA for both alpha and alpha 3 isoforms of Na+,K+-ATPase; mRNA was not detected in several other cell lines, including K562 and U937; no cell lines expressed alpha+ mRNA. In uninduced HL60 cells, alpha 3 mRNA comprised 20-30% of total Na+,K+-ATPase mRNA. Furthermore, in HL60 and HEL cells, both alpha and alpha 3 mRNA declined after induction of maturation by DMSO, retinoic acid, or hemin. However, the reduction in alpha 3 mRNA was far more dramatic. alpha 3 mRNA virtually disappeared, but alpha mRNA declined by only approximately 50%. In contrast, when maturation of HL60 cells along the monocyte/macrophage lineage was induced by exposure to phorbol esters, alpha 3 mRNA remained abundant. Moreover, mRNA for the beta subunit of the Na+,K+-ATPase increased dramatically. Our results demonstrate that the alpha 3 isoform, formerly thought to be confined to neuromuscular tissues, is expressed in restricted lineages of hematopoietic origin. These leukemia cell lines should provide a useful model for analyzing regulation of the alpha 3 isoform gene and characterization of alpha 3 isoform activities.

Animals↗

Isoforms of human C4b-binding protein. I. Molecular basis for the C4BP isoform pattern and its variations in human plasma.

Human C4b-binding protein (C4BP) is an important regulator of the complement system that also binds and inactivates the anticoagulant vitamin K-dependent protein S. These two activities are performed by two distinct polypeptides of 70 kDa and 45 kDa known as alpha- and beta-chains, respectively. C4BP is present in plasma in various isoforms with different alpha beta composition. Here we report multiple discrete variations of the relative levels of the C4BP isoforms among normal individuals and provide evidence that they are determined by genetic factors that segregate with the regulator of complement activation gene cluster. We also report the characterization of the C4BP molecules secreted by HepG2 and Hep3B cells, as well as transfection experiments in COS cells, to illustrate that the relative levels of expression of the C4BPA and C4BPB genes play a major role in determining the proportion in which the different C4BP isoforms are synthesized. Altogether, the data indicate that the human C4BP isoform pattern is genetically determined, but can be modified by factors with a differential effect on the expression of the C4BPA and C4BPB genes. These observations provide a new way to explore the possible association between elevated levels of C4BP and an increased risk to thromboembolic disorders.

Animals↗

8S globulin of mungbean [Vigna radiata (L.) Wilczek]: cloning and characterization of its cDNA isoforms, expression in Escherichia coli, purification, and crystallization of the major recombinant 8S isoform.

Three isoforms of the cDNA of the major 8S globulin of mungbean, 8Salpha, 8Salpha', and 8Sbeta, were isolated, cloned, and characterized. The cDNA sequences of 8Salpha, 8Salpha', and 8Sbeta had open reading frames of 1362, 1359 or 1362, and 1359 bp, respectively, which code for 454, 453 or 454, and 453 amino acids corresponding to molecular weights of 51 973, 51 627 or 51 758, and 51 779, respectively. Homology in terms of cDNA and amino acid sequences was 91-92% between 8Salpha and 8Salpha', 87% between 8Salpha and 8Sbeta, and 86-88% between 8Salpha' and 8Sbeta. The signal peptide was found to be 1-25, 1-24 or 25, and 1-23 for 8Salpha, 8Salpha', and 8Sbeta, respectively, using the signalP website (Nielsen, H.; Engelbrecht, J.; Brunak, S.; von Heijne, G. Protein Eng. 1997, 10, 1-6). The propeptide was determined to be IVHREN. A single site for glycosylation (N-X-S/T) was observed about 90 amino acids from the C terminus. Homology between mungbean 8S isoforms and other 7-8S proteins ranged from 45 to 68% within members of the legume family and 29 to 34% for crops of different species. The major isoform 8Salpha was expressed in Escherichia coli and purified by successive ammonium sulfate fractionation, hydrophobic interaction, and Mono Q column chromatography. The recombinant 8Salpha, but not the native form, was successfully crystallized producing rhombohedral crystals.

Amino Acid Sequence↗

Functional specificity in 14-3-3 isoform interactions through dimer formation and phosphorylation. Chromosome location of mammalian isoforms and variants.

14-3-3 is now accepted as a novel type of dimeric protein that can modulate interaction between proteins involved in cell signalling and other functions. Target proteins that interact with 14-3-3 isoforms are involved in regulation of cell cycle, intracellular trafficking/targeting, signal transduction, cytoskeletal structure and transcription. In many cases, these proteins show a distinct preference for a particular isoform(s) of 14-3-3. A specific repertoire of dimer formation may influence which of the 14-3-3 interacting proteins could be brought together. The purpose of this review is to give an overview of mammalian 14-3-3 sequences, structures and post-translational modifications that may explain the known interactions with other proteins and mechanism(s). The regulation of interaction may involve phosphorylation of the interacting protein and in some cases the phosphorylation of 14-3-3 isoforms themselves.

14-3-3 Proteins↗

p53/p63/p73 isoforms: an orchestra of isoforms to harmonise cell differentiation and response to stress.

p63, p73 and p53 compose a family of transcription factors involved in cell response to stress and development. p53 is the most frequently mutated gene in cancer (50%) and loss of p53 activity is considered to be ubiquitous to all cancers. Recent publications may have a profound impact on our understanding of p53 tumour suppressor activity. p63, p73 and p53 genes have a dual gene structure conserved in drosophila, zebrafish and man. They encode for multiple p63, p73 or p53 proteins containing different protein domains (isoforms) due to multiple splicing, alternative promoter and alternative initiation of translation. In this review, we describe the different isoforms of p63, p73, p53 and their roles in development and cancer. The changes in the interactions between p53, p63 and p73 isoforms are likely to be fundamental to our understanding in the transition between normal cell cycling and the onset of tumour formation.

Alternative Splicing↗

The muscle isoform of 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase of the teleost Sparus aurata: relationship with the liver isoform.

The liver isoform of 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase of the teleost fish Sparaus aurata has several characteristics similar to the skeletal muscle isoform of mammals. In order to ascertain the relation between muscle and liver isoforms in teleost, 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase was purified from skeletal muscle of S. aurata. The muscle isozyme is composed of subunits with a molecular weight of 54 kDa, is bifunctional, and has an activity ratio kinase to bisphosphatase of 2.5. Muscle 6-phosphofructo 2-kinase is not sensitive to glycerol 3-phosphate inhibition and has noncooperative KmATP, higher than the liver isozyme. Thus, the kinetic characteristics of the muscle were distinguishable from the liver isozyme. Furthermore, the muscle isozyme is not a substrate of cAMP-dependent protein kinase. Despite those differences, two polyclonal antibodies raised against purified liver and muscle isozymes from S. aurata are not able to distinguish between them. Both antisera recognize with lower affinity recombinant rat liver 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase. A third antibody raised against the rat liver isozyme was also able to immunoprecipitate the teleost enzymes. The close immunological properties found suggest that S. aurata isozymes share epitopes in common. Considering the kinetic and immunological data reported, it is likely that the skeletal muscle/liver isozymes in teleost are products of a differentially spliced transcript of the same gene, as it is in rat. As those species are distant in vertebrate evolution, the similitude suggest that a common ancestral gene is involved in the muscle/liver 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase system in vertebrates.

Animals↗

The tissue distribution of mRNAs for the PACE4 isoforms, kexin-like processing protease: PACE4C and PACE4D mRNAs are major transcripts among PACE4 isoforms.

In the previous study [Biochem. Biophys. Res. Commun. (1994) 200, 943-950] we identified two novel cDNAs (PACE4C and PACE4D) encoding human Kexin-like protease, the PACE4 isoforms. In this study, we examined the expression of PACE4 isoform transcripts in various rat tissues. To detect very low levels and to distinguish among these isoforms, we used the reverse transcriptase-polymerase chain reaction (RT-PCR). PACE4C and PACE4D transcripts were detected in most tissues like PACE4A transcripts, however their tissue distribution profiles and the extent of expression differ. PACE4C and PACE4D transcripts are expressed at a much higher level than PACE4A transcript. These results indicate that PACE4C and PACE4D mRNAs are major transcripts of PACE4.

Animals↗

A novel pancreatic beta-cell isoform of calcium/calmodulin-dependent protein kinase II (beta 3 isoform) contains a proline-rich tandem repeat in the association domain.

There is evidence for a role for calcium/calmodulin-dependent protein phosphorylation in regulation of insulin secretion but the molecular nature of the kinase(s) responsible is unknown. In this study, the screening of a neonatal rat islet cDNA library resulted in the isolation of a 2 kb clone that was 99% homologous to the beta' isoform of calcium/calmodulin-dependent protein kinase II. The predicted 589 amino acid sequence with a calculated mass of 64,976 Da contained a 24 amino acid deletion in addition to the 15 amino acid deletion that differentiates the beta' from the beta isoform, and included an 86 amino acid novel domain consisting of a tandem repeat of proline-rich residues. The expression of this new isoform of calcium/calmodulin-dependent protein kinase II (beta 3) was confirmed in beta-cell lines and testis by DNA amplification of the sequence encoding the inserted domain by reverse transcriptase-polymerase chain reaction, followed by Southern analysis.

Alternative Splicing↗

Functional analysis of human PACE4-A and PACE4-C isoforms: identification of a new PACE4-CS isoform.

There are seven known subtilisin/kexin-like proprotein convertases responsible for the processing of numerous precursors at either pairs or specific single basic residues. Three members, PACE4, PC4 and PC5, exhibit alternative splicing of their RNAs resulting in the generation of multiple isoforms differing in their C- or N-terminal segments. In this study we examined the biosynthesis, functional activity and cellular localization of two of these isoforms, namely the full length PACE4-A and the C-terminally truncated PACE4-C which lacks 11 amino acids at the end of its chaperone-like P-domain. We report the existence of a new isoform, termed PACE4-CS, which is a C-terminally shortened version of PACE4-C. Cellular expression results demonstrated that PACE4-A codes for a functional secretable enzyme capable of cleaving pro7B2 into 7B2. In contrast, PACE4-CS is not secreted since it remains in the endoplasmic reticulum as an inactive zymogen form, thereby emphasizing the importance of the integrity of the P-domain. Microsequencing of the intracellular PACE4-CS protein in two cell lines revealed that it is proPACE4-CS with an N-terminal trimming reminiscent of the action of a dipeptidylpeptidase recognizing the motifs X-Ala and X-Pro.

Amino Acid Sequence↗

Ethanol oxidation into acetaldehyde by 16 recombinant human cytochrome P450 isoforms: role of CYP2C isoforms in human liver microsomes.

The involvement of cytochromes P450 (CYPs) in the oxidation of ethanol into acetaldehyde was investigated by using 16 recombinant human CYP isoforms. Apparent K(m) and V(m) were determined for CYP1A1, CYP1A2, CYP1B1, CYP2A6, CYP2B6, CYP2C8, CYP2C9*1, CYP2C9*2, CYP2C9*3, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP2J2, CYP3A4 and CYP4A11. All of the tested CYPs, except CYP2A6 and CYP2C18, metabolized ethanol into significant amounts of acetaldehyde and displayed K(m) values around 10mM. The significant correlation found between ethanol oxidation and CYP2E1, CYP3A4 and CYP1A2 catalytic activities in a panel of human liver microsomes confirmed the strong implication of these CYPs in ethanol metabolism. The contribution of CYP2C isoforms which are the most abundant in the liver after CYP3A4, was studied using selective inhibitors either with recombinant CYP2C isoforms or in human liver microsomes. Tienilic acid (100 microM) and ticlopidine (20 microM), mechanism-based inhibitors of CYP2C9 and CYP2C19, respectively, decreased ethanol oxidation by 8+/-1.2% and 7.6+/-1.6% in human liver microsomal samples while selective inhibitors of CYP2E1 (DEDTC 100 microM), CYP3A4 (TAO 50 microM) and CYP1A2 (furafylline 25 microM) decreased it by 11.9+/-2.1%, 19.8+/-1.9% and 16.3+/-3.9%, respectively. As ethanol can be metabolized by most of CYPs, it helps to explain or predict alcohol-xenobiotics interactions which are of high importance in medical prescription.

Acetaldehyde↗