A novel GTP-binding protein, Sar1p, is involved in transport from the endoplasmic reticulum to the Golgi apparatus.

TLDR

SAR1 is a novel 21‑kDa GTP‑binding protein isolated as a multicopy suppressor of the yeast ER‑Golgi transport mutant sec12, with sequence homology to ras‑related GTPases. The study aims to determine whether SAR1 collaborates with Sec12p to direct ER‑Golgi protein transport. The authors expressed SAR1 from a GAL1 promoter in a haploid strain lacking endogenous SAR1 to assess its function. Disruption of SAR1 is lethal, and conditional expression permits growth in galactose but causes arrest in glucose, with accumulation of ER‑resident precursors of alpha‑mating factor and carboxypeptidase Y, indicating a role in ER‑Golgi transport.

Abstract

SAR1, a gene that has been isolated as a multicopy suppressor of the yeast ER-Golgi transport mutant sec12, encodes a novel GTP-binding protein. Its nucleotide sequence predicts a 21-kD polypeptide that contains amino acid sequences highly homologous to GTP-binding domains of many ras-related proteins. Gene disruption experiments show that SAR1 is essential for cell growth. To test its function further, SAR1 has been placed under control of the GAL1 promoter and introduced into a haploid cell that had its chromosomal SAR1 copy disrupted. This mutant grows normally in galactose medium but arrests growth 12-15 h after transfer to glucose medium. At the same time, mutant cells accumulate ER precursor forms of a secretory pheromone, alpha-mating factor, and a vacuolar enzyme, carboxypeptidase Y. We propose that Sec12p and Sarlp collaborate in directing ER-Golgi protein transport.