Identification of SEC12, SED4, Truncated SEC16, and EKS1/HRD3 as Multicopy Suppressors of ts Mutants of Sari GTPase

Abstract

The yeast SARI gene encodes a low-molecular-weight GTPase which is essential for the formation of transport vesicles from the endoplasmic reticulum (ER). To understand how the Sarlp function is regulated in its GTPase cycle, we searched for multicopy suppressors of sar1 temperature-sensitive mutants and identified SEC12, SED4, truncated SEC16, and EKS1. EKSl turns out to be identical to HRD3, which was independently isolated as a gene implicated in the degradation of an HMG-CoA reductase isozyme, Hmg2p. In this paper, we show that the product of EKS1/HRD3 is a type-I transmembrane glycoprotein and resides in the ER. The eksl/hrd3 disrupted cells are normal in growth and transport of cargo proteins, but missecrete BiP (Kar2p). The overexpression of EKSl/HRD3, which stabilizes Hmg2p, did not affect the stability of wild-type or mutant Sarlp or any early Sec proteins we examined. These results suggest that the role of Ekslp/Hrd3p is not involved in the ER protein degradation in general but ratheir required for the maintenance of the ER membrane functions. The novel genetic interactions unveiled between SARI, SEC12, SEC16, and SED4 will provide useful information as to how the complex machinery of vesicle budding operates.

COPII vesicle, ER protein degradation, Saccharomyces cerevisiae, SARI, vesicular transport