Differential modes of orphan subunit recognition for the WRB/CAML complex

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 using a model system of the WRB/CAML complex: an essential insertase for tail-anchored proteins in the endoplasmic reticulum (ER). Our data suggests that the stability of each subunit is differentially regulated. In WRB9s absence, CAML folds incorrectly, causing aberrant exposure of a hydrophobic transmembrane domain to the cytosol. When present, WRB can post-translationally 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 at least two distinct regulatory pathways for the surveillance of orphan subunits during complex assembly in the mammalian ER.