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Two new Ypt GTPases are required for exit from the yeast trans-Golgi compartment.

Jedd G, Mulholland J, Segev N - J. Cell Biol. (1997)

Bottom Line: These observations suggest that Ypt31p and Ypt32p perform identical or overlapping functions.The ypt31/ 32 mutant secretory defect is clearly downstream from that displayed by a ypt1 mutant and is similar to that of sec4 mutant cells.Together, these results indicate that the Ypt31/32p GTPases are required for a step that occurs in the trans-Golgi compartment, between the reactions regulated by Ypt1p and Sec4p.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacological and Physiological Sciences, The University of Chicago, Illinois 60637, USA.

ABSTRACT
Small GTPases of the Ypt/rab family are involved in the regulation of vesicular transport. These GTPases apparently function during the targeting of vesicles to the acceptor compartment. Two members of the Ypt/rab family, Ypt1p and Sec4p, have been shown to regulate early and late steps of the yeast exocytic pathway, respectively. Here we tested the role of two newly identified GTPases, Ypt31p and Ypt32p. These two proteins share 81% identity and 90% similarity, and belong to the same protein subfamily as Ypt1p and Sec4p. Yeast cells can tolerate deletion of either the YPT31 or the YPT32 gene, but not both. These observations suggest that Ypt31p and Ypt32p perform identical or overlapping functions. Cells deleted for the YPT31 gene and carrying a conditional ypt32 mutation exhibit protein transport defects in the late exocytic pathway, but not in vacuolar protein sorting. The ypt31/ 32 mutant secretory defect is clearly downstream from that displayed by a ypt1 mutant and is similar to that of sec4 mutant cells. However, electron microscopy revealed that while sec4 mutant cells accumulate secretory vesicles, ypt31/32 mutant cells accumulate aberrant Golgi structures. The ypt31/32 phenotype is epistatic to that of a sec1 mutant, which accumulates secretory vesicles. Together, these results indicate that the Ypt31/32p GTPases are required for a step that occurs in the trans-Golgi compartment, between the reactions regulated by Ypt1p and Sec4p. This step might involve budding of vesicles from the trans-Golgi. Alternatively, Ypt31/32p might promote secretion indirectly, by allowing fusion of recycling vesicles with the trans-Golgi compartment.

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Model for the action of Ypt GTPases in the yeast exocytic secretory pathway. Ypt1p and Sec4p are essential for three  secretory steps: ER to cis-Golgi, cis- to medial-Golgi, and transGolgi to the plasma membrane (Novick et al., 1981; Jedd et al.,  1995). For two of these steps, it was shown that Ypt1 and Sec4  proteins function in targeting of ER- and trans-Golgi–derived  vesicles, respectively (Novick et al., 1981; Rexach and Schekman,  1991; Segev, 1991). (A) Ypt31/32 proteins are suggested to have a  role in promoting vesicle budding from the trans-Golgi. (B) Alternatively, Ypt31/32 may regulate a fusion step between the  trans-Golgi and retrograde vesicles derived from a recycling compartment (see Discussion).
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Figure 12: Model for the action of Ypt GTPases in the yeast exocytic secretory pathway. Ypt1p and Sec4p are essential for three secretory steps: ER to cis-Golgi, cis- to medial-Golgi, and transGolgi to the plasma membrane (Novick et al., 1981; Jedd et al., 1995). For two of these steps, it was shown that Ypt1 and Sec4 proteins function in targeting of ER- and trans-Golgi–derived vesicles, respectively (Novick et al., 1981; Rexach and Schekman, 1991; Segev, 1991). (A) Ypt31/32 proteins are suggested to have a role in promoting vesicle budding from the trans-Golgi. (B) Alternatively, Ypt31/32 may regulate a fusion step between the trans-Golgi and retrograde vesicles derived from a recycling compartment (see Discussion).

Mentions: In this study, we show that two novel GTPases, Ypt31p and Ypt32p, have a role in the yeast exocytic pathway, in a step between those regulated by two other Ypt/rab GTPases, Ypt1p and Sec4p. The sequence similarity between Ypt31p and Ypt32p, and the fact that the presence of only one of them is required for cell viability, strongly suggests they are functional homologues. We propose that Ypt31p and Ypt32p function in exit from the trans-Golgi compartment (Fig. 12). This conclusion is based on phenotypic analysis of inactivating mutations in these genes, a comparison of ypt1, ypt31/32, and sec4 mutant strains by electron microscopy and epistasis analysis using the ypt31/32 mutant and a late secretory sec1 mutant.


Two new Ypt GTPases are required for exit from the yeast trans-Golgi compartment.

Jedd G, Mulholland J, Segev N - J. Cell Biol. (1997)

Model for the action of Ypt GTPases in the yeast exocytic secretory pathway. Ypt1p and Sec4p are essential for three  secretory steps: ER to cis-Golgi, cis- to medial-Golgi, and transGolgi to the plasma membrane (Novick et al., 1981; Jedd et al.,  1995). For two of these steps, it was shown that Ypt1 and Sec4  proteins function in targeting of ER- and trans-Golgi–derived  vesicles, respectively (Novick et al., 1981; Rexach and Schekman,  1991; Segev, 1991). (A) Ypt31/32 proteins are suggested to have a  role in promoting vesicle budding from the trans-Golgi. (B) Alternatively, Ypt31/32 may regulate a fusion step between the  trans-Golgi and retrograde vesicles derived from a recycling compartment (see Discussion).
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2139891&req=5

Figure 12: Model for the action of Ypt GTPases in the yeast exocytic secretory pathway. Ypt1p and Sec4p are essential for three secretory steps: ER to cis-Golgi, cis- to medial-Golgi, and transGolgi to the plasma membrane (Novick et al., 1981; Jedd et al., 1995). For two of these steps, it was shown that Ypt1 and Sec4 proteins function in targeting of ER- and trans-Golgi–derived vesicles, respectively (Novick et al., 1981; Rexach and Schekman, 1991; Segev, 1991). (A) Ypt31/32 proteins are suggested to have a role in promoting vesicle budding from the trans-Golgi. (B) Alternatively, Ypt31/32 may regulate a fusion step between the trans-Golgi and retrograde vesicles derived from a recycling compartment (see Discussion).
Mentions: In this study, we show that two novel GTPases, Ypt31p and Ypt32p, have a role in the yeast exocytic pathway, in a step between those regulated by two other Ypt/rab GTPases, Ypt1p and Sec4p. The sequence similarity between Ypt31p and Ypt32p, and the fact that the presence of only one of them is required for cell viability, strongly suggests they are functional homologues. We propose that Ypt31p and Ypt32p function in exit from the trans-Golgi compartment (Fig. 12). This conclusion is based on phenotypic analysis of inactivating mutations in these genes, a comparison of ypt1, ypt31/32, and sec4 mutant strains by electron microscopy and epistasis analysis using the ypt31/32 mutant and a late secretory sec1 mutant.

Bottom Line: These observations suggest that Ypt31p and Ypt32p perform identical or overlapping functions.The ypt31/ 32 mutant secretory defect is clearly downstream from that displayed by a ypt1 mutant and is similar to that of sec4 mutant cells.Together, these results indicate that the Ypt31/32p GTPases are required for a step that occurs in the trans-Golgi compartment, between the reactions regulated by Ypt1p and Sec4p.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacological and Physiological Sciences, The University of Chicago, Illinois 60637, USA.

ABSTRACT
Small GTPases of the Ypt/rab family are involved in the regulation of vesicular transport. These GTPases apparently function during the targeting of vesicles to the acceptor compartment. Two members of the Ypt/rab family, Ypt1p and Sec4p, have been shown to regulate early and late steps of the yeast exocytic pathway, respectively. Here we tested the role of two newly identified GTPases, Ypt31p and Ypt32p. These two proteins share 81% identity and 90% similarity, and belong to the same protein subfamily as Ypt1p and Sec4p. Yeast cells can tolerate deletion of either the YPT31 or the YPT32 gene, but not both. These observations suggest that Ypt31p and Ypt32p perform identical or overlapping functions. Cells deleted for the YPT31 gene and carrying a conditional ypt32 mutation exhibit protein transport defects in the late exocytic pathway, but not in vacuolar protein sorting. The ypt31/ 32 mutant secretory defect is clearly downstream from that displayed by a ypt1 mutant and is similar to that of sec4 mutant cells. However, electron microscopy revealed that while sec4 mutant cells accumulate secretory vesicles, ypt31/32 mutant cells accumulate aberrant Golgi structures. The ypt31/32 phenotype is epistatic to that of a sec1 mutant, which accumulates secretory vesicles. Together, these results indicate that the Ypt31/32p GTPases are required for a step that occurs in the trans-Golgi compartment, between the reactions regulated by Ypt1p and Sec4p. This step might involve budding of vesicles from the trans-Golgi. Alternatively, Ypt31/32p might promote secretion indirectly, by allowing fusion of recycling vesicles with the trans-Golgi compartment.

Show MeSH
Related in: MedlinePlus