The presence of a Sar1 gene family in Brassica campestris that suppresses a yeast vesicular transport mutation Sec12-1.

Two new members (Bsar1a and Bsar1b) of the Sar1 gene family have been identified from a flower bud cDNA library of Brassica campestris and their functional characteristics were analyzed. The two clones differ from each other at 14 positions of the 193 amino acid residues deduced from their coding region. The amino acid sequences of Bsar1a and Bsar1b are most closely related to the Sar1 family, genes that function early in the process of vesicle budding from the endoplasmic reticulum (ER). The sequences contain all the conserved motifs of the Ras superfamily (G1-G4 motifs) as well as the distinctive structural feature near the C-terminus that is Sar1 specific. Our phylogenetic analysis confirmed that these two clones can indeed be considered members of the Sar1 family and that they have a close relationship to the ARF family. The Bsar1 proteins, expressed in Escherichia coli, cross-reacted with a polyclonal antibody prepared against Saccharomyces cerevisiae Sar1 protein. It also exhibited GTP-binding activity. Genomic Southern blot analysis, using the 3'-gene-specific regions of the Bsar1 cDNAs as probes, revealed that the two cDNA clones are members of a B. campestris Sar1 family that consists of 2 to 3 genes. RNA blot analysis, using the same gene-specific probes, showed that both genes are expressed with similar patterns in most tissues of the plant, including leaf, stem, root, and flower buds. Furthermore, when we placed the two Bsar1 genes under the control of the yeast pGK1 promoter into the temperature-sensitive mutant yeast strain S. cerevisiae Sec12-1, they suppressed the mutation which consists of a defect in vesicle transport. The amino acid sequence similarity, the GTP-binding activity, and the functional suppression of the yeast mutation suggest that the Bsar1 proteins are functional homologues of the Sar1 protein in S. cerevisiae and that they may perform similar biological functions.

Isolation of an additional soybean cDNA encoding Ypt/Rab-related small GTP-binding protein and its functional comparison to Sypt using a yeast ypt1-1 mutant.

We have previously reported the isolation of a gene from a soybean cDNA library encoding a Ypt/Rab-related small GTP-binding protein, Sypt. Here, we report the isolation of a second Ypt/Rab-related gene, designated Srab2, from the same soybean cDNA library. And we compare the in vivo function of the two soybean genes utilizing a yeast ypt1-1 mutant. The Srab2 gene encodes 211 amino acid residues with a molecular mass of 23 169 Da. The deduced amino acid sequence of the Srab2 is closely related to the rat (76%) and human (75%) Rab2 proteins, but it shares relatively little homology to Sypt (46%) and Saccharomyces cerevisiae ypt proteins (41%). Genomic Southern blot analysis using the cDNA insert of Srab2 revealed that it belongs to a multigene family in the soybean genome. The protein encoded by Srab2 gene, when expressed in Escherichia coli, disclosed a GTP-binding activity. The expression pattern of the Srab2 gene is quite different from that of the Sypt gene. The Srab2 gene is predominantly expressed in the plumule region, while expression was very low in the other areas in soybean seedlings. On the other hand, the Sypt mRNA is not detectable in any tissues of soybean seedlings grown in the dark. However, light significantly suppressed the Srab2 gene expression, but enhanced the transcript levels of the Sypt gene in leaf and, at even higher levels, in root tissues. When the Srab2 and Sypt genes are introduced separately into a S cerevisiae defective in vesicular transport function, the Srab2 gene cannot complement the temperature-sensitive yeast ypt1-1 mutation at all, in contrast to the Sypt gene. In conclusion, the difference of functional complementation of the yeast mutation together with differential expression of the two genes suggest that the in vivo roles of the Srab2 and Sypt genes may be different in soybean cells.