The WRB Subunit of the Get3 Receptor is Required for the Correct Integration of its Partner CAML into the ER

Hugo J F Carvalho,Andrea Del Bondio,Sara F Colombo,Francesca Maltecca,Nica Borgese

doi:10.1038/s41598-019-48363-2

Abstract

Calcium-modulating cyclophilin ligand (CAML), together with Tryptophan rich basic protein (WRB, Get1 in yeast), constitutes the mammalian receptor for the Transmembrane Recognition Complex subunit of 40 kDa (TRC40, Get3 in yeast), a cytosolic ATPase with a central role in the post-translational targeting pathway of tail-anchored (TA) proteins to the endoplasmic reticulum (ER) membrane. CAML has also been implicated in other cell-specific processes, notably in immune cell survival, and has been found in molar excess over WRB in different cell types. Notwithstanding the stoichiometric imbalance, WRB and CAML depend strictly on each other for expression. Here, we investigated the mechanism by which WRB impacts CAML levels. We demonstrate that CAML, generated in the presence of sufficient WRB levels, is inserted into the ER membrane with three transmembrane segments (TMs) in its C-terminal region. By contrast, without sufficient levels of WRB, CAML fails to adopt this topology, and is instead incompletely integrated to generate two aberrant topoforms; these congregate in ER-associated clusters and are degraded by the proteasome. Our results suggest that WRB, a member of the recently proposed Oxa1 superfamily, acts catalytically to assist the topogenesis of CAML and may have wider functions in membrane biogenesis than previously appreciated.

Highlights

As is the case for many multisubunit complexes, the levels of each subunit of the WRB/CAML complex are strictly dependent on the presence of the other one[7,8]
We suggest that WRB may function catalytically as a chaperone/insertase to allow the correct topogenesis of its partner, and that it may play a wider role in membrane biogenesis than assisting insertion of TA proteins via the TRC40 pathway
CAML and WRB are the two components of the mammalian TRC40 receptor, which plays a key role in the insertion of TA proteins into the endoplasmic reticulum (ER) membrane

Summary

Introduction

As is the case for many multisubunit complexes, the levels of each subunit of the WRB/CAML complex are strictly dependent on the presence of the other one[7,8]. In the case of CAML, our previous results indicated that the ratio of CAML to WRB varies in different cell types, and that, in all the cases analysed, CAML is in stoichiometric excess[7]. The purpose of this excess CAML might, on the one hand, be linked to TA protein biogenesis, e.g., the efficiency of recruitment of TRC40-TA complexes to the ER could be increased by additional CAML subunits, which are responsible for www.nature.com/scientificreports/. We suggest that WRB may function catalytically as a chaperone/insertase to allow the correct topogenesis of its partner, and that it may play a wider role in membrane biogenesis than assisting insertion of TA proteins via the TRC40 pathway

Methods

Results

Conclusion