The ras superfamily proteins

Structural and functional analysis of ypt2, an essential ras-related gene in the fission yeast Schizosaccharomyces pombe encoding a Sec4 protein homologue

Using the cloned Saccharomyces cerevisiae YPT1 gene as hybridization probe, a gene, designated ypt2, was isolated from the fission yeast Schizosaccharomyces pombe and found to encode a 200 amino acid long protein most closely related to the ypt branch of the ras superfamily. Disruption of the ypt2 gene is lethal. The bacterially produced ypt2 gene product is shown to bind GTP. A region of the ypt2 protein corresponding to but different from the 'effector region' of ras proteins is also different from that of ypt1 proteins of different species but identical to the 'effector loop' of the S.cerevisiae SEC4 gene product, a protein known to be required for vesicular protein transport. The S.pombe ypt2 gene under control of the S.cerevisiae GAL10 promoter is able to suppress the temperature-sensitive phenotype of a S. cerevisiae sec4 mutant, indicating a functional similarity of these GTP-binding proteins from the two very distantly related yeasts.

Identification of ras-related, YPT family genes in Schizosaccharomyces pombe

Screening for genes homologous to ras in Schizosaccharomyces pombe resulted in the isolation of a homolog of Saccharomyces cerevisiae YPT1. This S. pombe gene, named ypt3, has a coding capacity of 214 amino acids interrupted by two introns, and is essential for cell growth. Two more YPT1 homologs were isolated from S. pombe using a part of the ypt3 gene as the probe. One of them, named ypt1, is highly homologous to S. cerevisiae YPT1 and mouse ypt1 and is essential for cell growth. This gene has four introns and encodes 203 amino acids. Its cDNA placed downstream of the S. cerevisiae GAL7 promoter could complement S. cerevisiae ypt1-, indicating that Sp ypt1 and Sc YPT1 are functionally homologous. The other isolate, named ryh1, and a fourth homolog, ypt2, have been characterized by Gallwitz and co-workers. The ypt1, ypt2 and ypt3 genes, but not ryh1, constitute a family, their products having double cysteine as their C terminus and serine in place of a glycine residue highly conserved in ras proteins (mammalian Gly-12 or S. pombe Gly-17). The physiological roles of these genes appear to be distinct because each of them is indispensable for cell growth.

Characterization and expression of a Xenopus ras during oogenesis and development

We have characterized a cDNA which contains the entire coding sequence of a Xenopus laevis ras protein. The deduced amino acid sequence reveals a strong homology (92%) to human Ki-ras 2B protein. ras expression has been studied both qualitatively and quantitatively during Xenopus development. ras is expressed as a maternal mRNA in oocytes and early embryos at a level up to 1.5 x 10(7) copies per mature oocyte, corresponding to the level of ras mRNA found in 4 x 10(5) somatic growing cells. This level remains constant throughout the first rapid cleavage stages of the blastula before the midblastula transition (MBT). After this stage, the amount of ras RNA decreases gradually until the hatching tadpole stage, when a new zygotic expression is detected in the embryo. From that stage, a constitutive amount of 30-50 ras RNA transcripts per embryonic cell is registered, as observed in Xenopus proliferative somatic cells. The 23-kDa Xenopus ras protein has also been identified by both specific monoclonal antibody and in vitro transcription-translation experiments. It is expressed in oocytes before maturation, indicating that maturation is not the trigger for ras expression. The expression of Xenopus ras at a high level during oogenesis and early development suggests a major function of this gene both in meiosis and in mitosis events during embryonic development.

Identification of multiple ral gene products in human platelets that account for some but not all of the platelet Gn-proteins

Polyclonal antibodies raised against specific recombinant low molecular mass GTP-binding proteins were tested for their ability to recognize partially purified human platelet membrane Gn-proteins (i.e. proteins that bind [alpha-32P]GTP on nitrocellulose blots of SDS/polyacrylamide gels). An antiserum against simian ralA protein recognized a 27 kDa human platelet protein with the same apparent molecular mass as the major platelet Gn-protein (Gn27). In further analysis by two-dimensional polyacrylamide gel electrophoresis, the isoelectric focusing step permitted resolution of 12 major Gn-protein forms, seven of 27 kDa (Gn27a-g), one of 26 kDa (Gn26) and four of 24 kDa (Gn24a-d). The ralA antibody reacted strongly with the five most basic Gn27 species (a-e), weakly with Gn26 and not at all with Gn27f, Gn27g or Gn24a-d. We conclude that ral gene products account for some but probably not for all of the platelet Gn-proteins.

Chromosome mapping of the human ras-related rab3A gene to 19p13.2

The rab genes belong to one of the three main branches of the ras super family. The encoded rab proteins share 38 to 75% amino acid identity with the yeast YPT1 and SEC4 proteins. We used the human rab3A cDNA to map the corresponding gene on human chromosomes by chromosome sorting and in situ hybridization. Both techniques allowed the assignment of the rab3A gene to chromosome 19 with a regional localization on 19p13.2 obtained by in situ hybridization.

Biochemical properties of the YPT-related rab1B protein. Comparison with rab1A

We recently identified a novel rat cDNA: rab1B, closely related to the rab1A cDNA and to the yeast YPT1 gene. The rab1B cDNA encodes a 202 amino acid protein (22.1 kDa) that was produced in Escherichia coli under the control of the phi 10 promoter for the T7 RNA polymerase. The rab1B protein was purified in large amounts to near homogeneity in a simplified procedure. We studied the biochemical properties of rab1B and rab1A proteins. They both bind specifically GTP and GDP and possess intrinsic GTPase activities. The rab1B Lys21----Met mutant protein does not bind GTP, whereas the Ala65----Thr mutant has a reduced GTPase activity and is competent for autophosphorylation in the presence of GTP.

Low Mr GTP-binding proteins in human platelets: cyclic AMP-dependent protein kinase phosphorylates m22KG(I) in membrane but not c21KG in cytosol

We have purified and characterized two kinds of GTP-binding proteins with Mr of 22,000 in human platelet membrane (main; m22KG(I), minor; m22KG(II)) (Nagata, K. and Nozawa, Y. (1988) FEBS Lett. 238, 90-94). In this study, the main GTP-binding protein (m22KG(I)) was found to be phosphorylated by cyclic AMP-dependent protein kinase (A-kinase), but not by protein kinase C. About 0.5 mol of phosphate was maximally incorporated into one mol of the protein and this phosphorylation was inhibited in the presence of A-kinase inhibitor. Phosphorylation of m22KG(I) did not alter either its GTP-binding or GTPase activity. When m22KG(I) was incubated alone or in the presence of 100 microM guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) and then exposed to A-kinase, no significant changes in the level of phosphorylation were observed. On the other hand, the most abundant GTP-binding protein with Mr of 21,000 (c21KG) in human platelet cytosol, which was identified as a transformation suppressor gene product (rap 1 protein, smg p21 and Krev-1 protein), was not phosphorylated by A-kinase under the same condition. However, c21KG was phosphorylated by A-kinase after pretreatment with alkaline phosphatase.

Detection of low molecular mass GTP-binding proteins in chromaffin granules and other subcellular fractions of chromaffin cells

A homogenate of purified chromaffin cells was fractionated, after removal of the nuclear fraction, by sucrose density gradient ultracentrifugation. The presence and subcellular localization of low molecular mass GTP-binding proteins was explored by incubation of blots of proteins from different subcellular fractions with [alpha-32P]GTP in the presence of Mg2+. The fractions enriched in intact chromaffin granule markers, i.e. catecholamines, chromogranin A, chromogranin B and cytochrome b-561 were also enriched in labelled GTP-binding proteins. Two major labelled components of 23 and 29 kDa were rapidly detected by autoradiography. Traces of 26 and 27 kDa components were also present. These components were detectable in both plasma and granule membranes. In addition to these components, the cytosolic fraction contained another GTP-binding protein of about 20 kDa. Binding of [alpha-32P]GTP was specific and dependent on Mg2+. By analogy to the findings reported in non-mammalian systems, the observations described here suggest the involvement of low molecular mass GTP-binding proteins in the chromaffin cell secretory process.