3.A.19 The TMS Recognition/Insertion Complex (TRC) Family
An important class of proteins in eukaryotic cells includes tail-anchored (TA) membrane proteins, which include cytochrome b5 (the founding member of the TA family), the SNARE proteins involved in vesicle trafficking, proteins involved in apoptosis (Bcl-2 family), and several subunits of the mitochondrial and endoplasmic reticulum (ER) protein translocation channels. The yeast genome encodes 55 tail-anchored membrane proteins (Beilharz et al., 2003) that are ultimately localized to the nuclear envelope, the outer mitochondrial membrane, the peroxisome, and all membranes within the exocytic and endocytic pathways (Mandon and Gilmore, 2007).
TA proteins are integrated into membranes via a carboxy-terminal hydrophobic segment that serves both as a transmembrane-spanning domain and the targeting signal for initial insertion. Membrane insertion of the exocytic SNARE protein, synaptobrevin, requires ATP hydrolysis and one or more protease-sensitive ER membrane proteins (Kutay et al., 1995), not the SRP receptor or the Sec61 translocon.
Wild-type TRC, which exists in soluble and microsome-bound forms, is partially extracted from the ER by ATP. It is the targeting receptor for TA-protein insertion. Targeting of TRC to a membrane-bound TA-protein integrase is proposed to precede ATP-hydrolysis-dependent transfer of the nascent TA protein to the integrase.
Tail-anchored (TA) proteins serve numerous essential roles in cells. TRC40/Asna-1 interacts posttranslationally with TA proteins in a TMS-dependent manner for delivery to a proteinaceous receptor. Subsequent release from TRC40/Asna-1 and insertion into the membrane depends on ATP hydrolysis. Consequently, an ATPase-deficient mutant of TRC40/Asna-1 dominantly inhibited TA protein insertion selectively without influencing other translocation pathways (Stefanovic and Hegde, 2007).
TA proteins are post-translationally targeted to and inserted into the ER membrane through their single C-terminal transmembrane domain. Membrane insertion of TA proteins in mammalian cells is mediated by the ATPase TRC40/Asna1 (Get3 in yeast) and a receptor in the ER membrane. Vilardi et al. (2011) identified the tryptophan-rich basic protein (WRB), also known as congenital heart disease protein 5 (CHD5), as the ER membrane receptor for TRC40/Asna1. WRB shows sequence similarity to Get1, a subunit of the membrane receptor complex for yeast Get3. It is an ER-resident membrane protein that interacts with TRC40/Asna1 and recruits it to the ER membrane. A coiled-coil domain of WRB is the binding site for TRC40/Asna1. A soluble form of the coiled-coil domain interferes with TRC40/Asna1-mediated membrane insertion of TA proteins (Vilardi et al., 2011).
The reaction mediated by TRC40 is:
Tail-anchored protein (TA) (cytosol) ATP → TA protein (endomembrane; integrated) ADP Pi