In vivo analysis of amino acid permease folding in yeast

Sammanfattning: Plasma membrane (PM) proteins are critical for cells to respond to environmental cues, such as the availability of nutrients. The yeast Saccharomyces cerevisiae is able to sense extracellular amino acids using the SPS sensing system. Activation of the multimeric PM-localized SPS(Ssy1-Ptr3-Ssy5)-sensor complex occurs upon binding of external amino acids to Ssy1, inducing a conformational change. In a Ptr3-mediated event, the catalytic activity of the Ssy5 endoprotease is unfettered, leading to the proteolytic processing of two latent transcription factors, Stp1 and Stp2. Ssy1, the primary sensor component, is a non-transporting member of the amino acid permease (AAP) family of transport proteins, a family of eighteen complex integral membrane proteins comprised of 12 transmembrane segments (TMS). The AAPs exhibit a common requirement for the endoplasmic reticulum (ER)-localized membrane chaperone Shr3 to fold and to be transported to the PM. The absence of Shr3 leads to the accumulation of misfolded AAP species that are targeted for ER-associated degradation. Thus, proper Shr3 function is required as the most upstream and most downstream component of the SPS sensing system. In paper I, we investigate the chaperone function of Shr3. We report a surprisingly low level of sequence specificity underlies Shr3-AAP interactions. We used a split-ubiquitin approach to probe Shr3-AAP interactions in vivo. The Shr3-AAP interactions initiate early after the first two-to-four TMS of AAPs insert into the ER membrane, successively strengthening and then diminishing after all 12 TMS partition into the membrane. In paper II, we clarified the localization and trafficking determinants of Ssy1. A study by Kralt et al. 2015 reported that Ssy1 primarily localizes to the ER and is sorted to ER-PM tethers. These reported findings are clearly incompatible with the accepted model of amino acid sensing by the SPS-sensor. We critically re-examined the localization of Ssy1 and found that it indeed localizes to the PM, and importantly does so independent of ER-PM tethers. We also identified a novel ER exit motif in the carboxy-terminal tail of Ssy1 required for proper PM localization and SPS-sensor function. In paper III, we report that Ssy5 is able to cleave substrates in unusual contexts, i.e., an engineered substrate carrying rearranged recognition and cleavage determinants placed ectopically at the carboxy terminus of Stp1, and an ER-anchored substrate with Stp1 fused to the carboxy terminus of Shr3. Strikingly, Ssy5 catalyzed cleavage of Shr3-Stp1 in cells lacking ER-PM tethers, indicating that once activated, Ssy5 can find and cleave substrates distant from the PM. Consequently, cells must be able to rein in the activity of the Ssy5 protease to prevent spurious and improper proteolysis. Consistent with this notion, we report that the catalytic domain of Ssy5 is ubiquitylated in a Ptr3 and Yck1/2 dependent manner, and under amino acid-inducing conditions is subject to degradation. We propose a model that degradation of the Ssy5 catalytic domain is essential for resetting the SPS sensing system and a requisite for cells to regain the ability to correctly sense extracellular amino acids.