3.A.19.1.1
The ATP hydrolysis-dependent TRC receptor TRC40 (Asna-1) which functions as a chaparone protein, feeding into the WRB/CAML transporter complex (McDowell et al. 2020; Stefanovic and Hegde, 2007). TRC40 is homologous to the ArsA ATPase of E. coli (TC# 3.A.4.1.1) and the GET3 ATPase of yeast (TC# 8.A.26.1.1). Loss yields embryonic lethality. Tryptophan-rich basic protein (WRB) is the tail-anchored (TA) protein insertion receptor, also called congenital heart disease protein-5 (CHD5). It is related to the yeast Get1 protein in 3.A.21.1.1. Calcium-modulating cyclophilin ligand (CAML) is a mammal-specific receptor for TRC40, an ATPase targeting newly synthesized TA proteins.  CAML mediates membrane insertion of TA proteins.  TRC40 (Asna1) has been shown to mediate membrane insertion of two proteins, RAMP4 and Sec61beta, without the participation of other cytosolic proteins by a mechanism that depends on the presence of ATP or ADP and a protease-sensitive receptor in the ER membrane (Favaloro et al. 2010).  TRC40 is required for release of Herpes simplex virus 1 (HSV1) virions (Ott et al. 2016). The functions and reciprocal interactions of the two subunits of the heteromeric TRC40 recpeptor, WBR and CAML (CAMLG), have revealed mutual dependencies for stability; CAML seems to normally be present in 5-fold excess over WBR (Colombo et al. 2016). CAMLG interacts with Classical Swine Fever Virus (CSFV) p7 and mediates calcium permeability in the ER (Gladue et al. 2018). Retro-2 (Morgens et al. 2019) protects cells from ricin and Shiga-like toxin toxicity by inhibiting ASNA1-mediated ER targeting and insertion of tail-anchored proteins (Morgens et al. 2019). Get3 (TRC40) binding to the membrane insertase supports heterotetramer formation, and phosphatidylinositol binding at the heterotetramer interface stabilizes the insertase for efficient TA insertion in vivo. McDowell et al. 2020 identified a Get2/CAML cytoplasmic helix that forms a "gating" interaction with Get3/TRC40, important for TA insertion. Structural homology with YidC and the ER membrane protein complex (EMC) suggests an evolutionarily conserved insertion mechanism for divergent substrates utilizing a hydrophilic groove (McDowell et al. 2020).  The WRB subunit of the Get3 receptor is required for the correct integration of its partner CAML into the ER (Carvalho et al. 2019). Close coordination between chaperones is essential for the delivery of tail-anchored (TA) proteins containing a single C-terminal TMS to the ER by the GET pathway. For successful targeting, nascent TA proteins must be promptly chaperoned and loaded onto the cytosolic ATPase Get3 through a transfer reaction involving the chaperone SGTA and bridging factors, Get4, Ubl4a and Bag6. Keszei et al. 2021 reported cryo-EM structures of metazoan pretargeting GET complexes at 3.3-3.6 Å resolution. Get3 helix 8 and the Get4 C terminus form a composite lid over the Get3 substrate-binding chamber that is opened by SGTA. Another interaction with Get4 prevents formation of Get3 helix 4, which links the substrate chamber and ATPase domain. Both interactions facilitate TA protein transfer from SGTA to Get3 (Keszei et al. 2021).