Functional characterization of the yeast ER packaging chaperone Shr3p

Abstract

All living cells are enclosed by a plasma membrane (PM). The PM is a dynamic structure comprised of lipids and proteins that establishes and maintains the integrity of the cell. Cells utilize the PM to regulate metabolism by selectively altering the flow of metabolites entering the cell. The integral protein components of the PM, e.g., metabolite transport systems, are co- translationally inserted into the membrane of the endoplasmic reticulum (ER). Subsequent to membrane insertion, these proteins must fold properly to attain native conformations, a process that often requires specific processing events. At an early stage in the secretory pathway, integral PM and secreted proteins are transported from the ER to the Golgi via ER-derived transport vesicles. In vitro studies have shown that a set of cytosolic proteins (Sar1p, Sec23p/Sec24p complex, and Sec13p/Sec31p complex) coordinately function to catalyze the formation of ER transport vesicles. These components are recruited to the ER membrane and subsequently oligomerize to form a vesicle coat structure known as COPII Cargo proteins are separated from resident ER proteins concomitantly with COPII vesicle formation.

Amino acid uptake in yeast is mediated by general and specific amino acid permeases (aaps). Complete sequencing of the yeast genome has revealed the existence of 18 aaps that share extensive sequence homology. These permeases are integral polytopic membrane proteins comprised of twelve hydrophobic domains. Aaps require Shr3p, a 23 kDa membrane component of the ER, to be functionally expressed. In cells lacking Shr3p, aaps accumulate in the ER and are not transported to the plasma membrane. The ER export block observed in shr3 null mutants is specific for permeases, other plasma membrane proteins, secretory proteins and vacuolar proteins are processed and targeted correctly.

This thesis documents experiments that examine the molecular basis of Shr3p function. The membrane topology and folding of the general aap (GAP]) in SHR3 and shr3alpha null mutant cells was determined. No detectable differences in the orientation of transmembrane spanning domains was found, and shr3alpha null mutants do not express enhanced levels of ER stress response proteins even when aap expression is derepressed. These findings indicate that the folding and membrane topology of Gap1p is normal and thus independent of Shr3p function. Shr3p physically associates with Gap1p, but not with other polytopic membrane proteins such as Sec61p, Ga12p or Pma1p, in a transient complex that can be purified from N-dodecylmaltoside solubilized membranes. The COPII coatomer components See 1 3p, Sec23p, Sec24p, and Sec3 1p, ut not Sar1p, bind Shr3p via interactions with its carboxy-terminal domain. By facilitating the membrane association of COPII coatomer components in close association with aaps, Shr3p is likely to function as a primer of ERderived transport vesicle formation that directs the assembly of vesicle buds around aaps.

In SHR3 wild-type cells the degradation of Gap1p and the histidine specific permease (Hip1p) is dependent upon PEP4, demonstrating that in these cells aaps exit the ER and are degraded in the vacuole. In shr3alpha cells however, Gap1p and Hip1p are more stable and their degradation occurs independently of PEP4. Instead, in the absence of Shr3p, the degradation of Gap1p and Hip1p is dependent on UBC7, a gene that encodes an E2 ubiquitin conjugating enzyme required for proper ER associated degradation (ERAD). The degradation of well characterized ERAD substrates CPY* and Pdr5 * was decreased in shr3 null mutants, suggesting that the accumulation of aaps in the ER saturates a limiting ERAD component. In contrast the degradation rate of Fur4*, a mutant version of the uracil permease, was not affected in shr3 null mutants. Additionally, Hip1p was significantly more stable than Gap1p in cells lacking Hrd1p/Der3p, the ER localized membrane bound E3 ubiquitin protein-ligase. These latter results demonstrate that despite extensive sequence and structural similarities, and common requirement for Shr3p, different aaps are degraded by alternative ERAD dependent processes.