title: Membrane chaperone Shr3 assists in folding amino acid permeases preventing precocious ERAD.
abstract: The yeast endoplasmic reticulum (ER) membrane-localized chaperone Shr3 plays a critical role in enabling amino acid permeases (AAPs) to fold and attain proper structures required for functional expression at the plasma membrane. In the absence of Shr3, AAPs specifically accumulate in the ER, where despite the correct insertion of their 12 transmembrane segments (TMSs), they aggregate forming large molecular weight complexes. We show that Shr3 prevents aggregation and facilitates the functional assembly of independently coexpressed N- and C-terminal fragments of the general AAP Gap1. Shr3 interacts with and maintains the first five TMSs in a conformation that can posttranslationally assemble with the remaining seven TMSs. We also show that Doa10- and Hrd1-dependent ER-associated degradation (ERAD) pathways redundantly degrade AAP aggregates. In combination, doa10Delta hrd1Delta mutations stabilize AAP aggregates and partially suppress amino acid uptake defects of shr3 mutants. Consequently, in cells with impaired ERAD, AAPs are able to attain functional conformations independent of Shr3. These findings illustrate that folding and degradation are tightly coupled processes during membrane protein biogenesis.
authors.affiliation: Ludwig Institute for Cancer Research, S-17177 Stockholm, Sweden.
authors.forename: Jhansi
authors.initials: J
authors.lastname: Kota
authors.forename: C Fredrik
authors.initials: CF
authors.lastname: Gilstring
authors.forename: Per O
authors.initials: PO
authors.lastname: Ljungdahl
mesh_terms: D026905:Amino Acid Transport Systems / Q000737:chemistry / Q000378:metabolism*; D026921:Amino Acid Transport Systems, Neutral / Q000737:chemistry / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism; D033921:Vesicular Transport Proteins
chemical_list: C420623:AGP1 protein, S cerevisiae; D026905:Amino Acid Transport Systems; D026921:Amino Acid Transport Systems, Neutral; C491145:GAP1 protein, S cerevisiae; D008565:Membrane Proteins; C077883:SHR3 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; C104621:HRD1 protein, S cerevisiae; C091362:SSM4 protein, S cerevisiae; D044767:Ubiquitin-Protein Ligases