Capture and delivery of tail-anchored proteins to the endoplasmic reticulum

Abstract

Tail-anchored (TA) proteins fulfill diverse cellular functions within different organellar membranes. Their characteristic C-terminal transmembrane segment renders TA proteins inherently prone to aggregation and necessitates their posttranslational targeting. The guided entry of TA proteins (GET in yeast)/transmembrane recognition complex (TRC in humans) pathway represents a major route for TA proteins to the endoplasmic reticulum (ER). Here, we review important new insights into the capture of nascent TA proteins at the ribosome by the GET pathway pretargeting complex and the mechanism of their delivery into the ER membrane by the GET receptor insertase. Interestingly, several alternative routes by which TA proteins can be targeted to the ER have emerged, raising intriguing questions about how selectivity is achieved during TA protein capture. Furthermore, mistargeting of TA proteins is a fundamental cellular problem, and we discuss the recently discovered quality control machineries in the ER and outer mitochondrial membrane for displacing mislocalized TA proteins.

Figure 1.

TA protein targeting to the ER in yeast. Nascent TA proteins emerging from the ribosome can be captured by alternative ER-targeting machineries. A major route to the ER is via the GET pathway, involving ribosome-associated capture by Sgt2, followed by Get4/Get5-mediated handover to the Get3 ATPase and insertion into the ER membrane by a heterotetrameric GET receptor complex composed of Get1 and Get2.

Figure 2.

TA protein targeting to the ER in mammals. Mammalian TA proteins are predominantly targeted to the ER by the TRC pathway. After capture by SGTA, together with the BAG6 complex (BAG6, UBL4A, and TRC35), the TA protein is passed to the TRC40 chaperone for delivery to the ER-bound receptor complex formed by WRB and CAML. BAG6 has dual functions bridging ER targeting and ubiquitination of TA proteins and can be antagonized by SGTA. Ubiquitinated TA proteins can be deubiquitinated by ER-associated UPS20/UPS33.

Figure 3.

Architecture of the Get1/WRB and Get2/CAML in the GET/TRC receptor complex. Get1/WRB and Get2/CAML both possess three TMDs (labeled 1–3) and rely on each other for stability and correct assembly within the ER membrane. The cytosolic regions of Get2/CAML (N-terminus and a loop between TMD 2 and 3) and a cytosolic region of Get1/WRB between TMDs 1 and 2 are docking sites for Get3/TRC40 carrying a TA protein cargo. A hydrophilic groove formed by the Get1/WRB TMDs and Get2/CAML TMD3 is proposed to serve as an insertion route for TA proteins to enter the membrane. Assembly of the heterodimer shown here into the final heterotetrameric structure of the receptor upon Get3/TRC40 binding involves protein–lipid interactions.

Figure 4.

Quality control machineries regulating distribution of mislocalized TA proteins between the ER and other organelles. ER-destined and peroxisome (Perox.)-destined TA proteins can mislocalize to the OMM, and OMM TA proteins can mislocalize to the ER. ATP-dependent machines in these membranes can recognize and displace mistargeted protein while correctly localized proteins are retained, for example by interactions with binding partners (indicated by unnamed, colored circles on the OMM). Nomenclature is as in yeast.