Differential Modes of Orphan Subunit Recognition for the WRB/CAML Complex

• Published in: Cell reports (Cambridge) Vol. 30; no. 11; pp. 3691 - 3698.e5
• Main Authors: Inglis, Alison J., Page, Katharine R., Guna, Alina, Voorhees, Rebecca M.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 17.03.2020; Elsevier
• Subjects: Adaptor Proteins, Signal Transducing - metabolism; Animals; Dogs; endoplasmic reticulum; Endoplasmic Reticulum - metabolism; HEK293 Cells; Humans; membrane proteins; Multiprotein Complexes - metabolism; Nuclear Proteins - metabolism; oligomeric assembly; Protein Binding; Protein Stability; Protein Subunits - metabolism; Proteolysis; quality control; Rabbits; Ribosomes - metabolism; oligomeric assembly; membrane proteins; quality control; endoplasmic reticulum
• ISSN: 2211-1247; 2211-1247
• DOI: 10.1016/j.celrep.2020.02.084

A large proportion of membrane proteins must be assembled into oligomeric complexes for function. How this process occurs is poorly understood, but it is clear that complex assembly must be tightly regulated to avoid accumulation of orphan subunits with potential cytotoxic effects. We interrogated assembly in mammalian cells by using the WRB/CAML complex, an essential insertase for tail-anchored proteins in the endoplasmic reticulum (ER), as a model system. Our data suggest that the stability of each subunit is differentially regulated. In WRB’s absence, CAML folds incorrectly, causing aberrant exposure of a hydrophobic transmembrane domain to the ER lumen. When present, WRB can correct the topology of CAML both in vitro and in cells. In contrast, WRB can independently fold correctly but is still degraded in the absence of CAML. We therefore propose that there are at least two distinct regulatory pathways for the surveillance of orphan subunits in the mammalian ER. [Display omitted] •Unassembled subunits of membrane protein complexes must be recognized and degraded•The obligate hetero-oligomer WRB/CAML has differential modes of orphan recognition•WRB is inserted correctly independently of CAML but is degraded when unassembled•CAML requires WRB to fold correctly, which prevents exposure of a degron Most membrane proteins assemble into multi-subunit complexes. How unassembled subunits are recognized and triaged for degradation is poorly understood. Inglis et al. use the WRB/CAML complex to define two modes of orphan recognition: CAML folds incorrectly without WRB, exposing a degron, while WRB inserts correctly but is degraded when unassembled.