3.A.21 The C-terminal Tail-Anchored Membrane Protein Biogenesis/ Insertion Complex (TAMP-B) Family of the Guided Entry of Tail-anchored Protein (GET) Superfamily
Sophisticated mechanisms have evolved to target eukaryotic membrane proteins to the correct membrane-surrounded organelle and to protect them from aggregation. Of central importance are cytosolic factors that decode the targeting information of a signal sequence by transient association and escort membrane proteins to their designated location (Schuldiner et al., 2008). Tail-anchored membrane proteins (TA proteins) consist of an N-terminal soluble domain and a single C-terminal transmembrane segment (TMS) (Borgese et al., 2007). The C-terminal localization of their targeting signal requires release of the synthesized protein from the ribosome before interaction with an insertion machinery (Cross et al., 2009). This precludes TA proteins from cotranslational insertion mediated by the signal recognition particle (SRP)-Sec61 translocon system (TC# 3.A.5), which mainly targets membrane proteins with an N-terminal signal sequence. The tail-anchored assemblly has been studied in unicellular eukaryotes such as Toxoplasma gondii (Padgett et al. 2016).
Although some studies suggest an unassisted insertion of TA proteins into the mitochondrial outer membrane (Meineke et al., 2008), most endoplasmic reticulum (ER)-destined TA proteins are thought to insert by an energy-dependent process, which involves several cytosolic factors. In yeast, it has been shown that the adenosine triphosphatase (ATPase) Get3 (guided-entry of TA proteins-3 pathway) is necessary for the biogenesis of TA proteins (Schuldiner et al., 2008). Get3 is a dimeric protein with each subunit comprising a nucleotide-binding domain (NBD) and a methionine-rich α-helical domain that has been implicated in TA protein binding (TA binding domain, TABD). Mateja et al. 2015 reconstituted the assembly pathway for a functional targeting complex and showed that it comprises a TA protein bound to a Get3 homodimer. Crystal structures of Get3 bound to different TA proteins showed an α-helical TMS occupying a hydrophobic groove that spans the Get3 homodimer.
Crystal structures of Get3 have shown that the protein switches between open and closed states, depending on its nucleotide load. Whereas in the apo and magnesium-free adenosine diphosphate (ADP) forms, the open state is favored (Bozkurt et al., 2009), ADP-Mg2+ and the nonhydrolyzable ATP analog 5′-adenylyl-β,γ-imidodiphosphate-Mg2+ (AMPPNP-Mg2+) induce the closed state, which is further tightened up in the transition state of adenosine triphosphate (ATP) hydrolysis (Mateja et al., 2009). The hydrophobic groove responsible for TA binding seems fully assembled only in the transition state. At the membrane, Get3 interacts with the two receptor proteins, Get1 and Get2, which are essential for TA protein insertion (Schuldiner et al., 2008).
Tail-anchored (TA) proteins in yeast contain a C-terminal membrane anchor and are posttranslationally delivered to the endoplasmic reticulum (ER) membrane by the Get3 adenosine triphosphatase (an ArsA homologue) interacting with the hetero-oligomeric 3 TMS Get1/2 membrane receptor. Stefer et al. (2011) have determined crystal structures of Get3 in complex with the cytosolic domains of Get1 and Get2 in different functional states. The heterotetrameric Get1/Get2 complex stoichiometry is (Get1)2(Get2)2. The structural data, together with biochemical experiments, show that Get1 and Get2 use adjacent, partially overlapping binding sites and that both can bind simultaneously to Get3. Docking to the Get1/2 complex allows for conformational changes in Get3 that are required for TA protein insertion. A molecular mechanism for nucleotide-regulated delivery of TA proteins was proposed (Stefer et al., 2011).
GET3 cooperates with the HDEL receptor ERD2 to mediate ATP-dependent retrieval of ER proteins that contain a C-terminal HDEL sequence (Schuldiner et al., 2005). This sequence is the retention signal from the Golgi to the ER. It may also be involved in low-level resistance to oxyanions such as arsenite, and in heat tolerance (Shen et al., 2003). It interacts with the Gef1 Cl- transporter in a copper-dependent fashion (Metz et al., 2006). Metz et al. claimed that the arsenite binding site of the E. coli ArsA is not conserved in Arr4p. The homodimer binds to the C-terminus of Gef1p. Both Gef1p and GET3 are required for normal growth under iron-limiting conditions. gef1 mutants lose high-affinity iron uptake because the Fet3p multi-copper oxidase involved in iron uptake (TC #1.A.11.1.1) does not mature normally in a gef1 mutant. This is because copper loading of Fet3p in the lumen of the late secretory pathway requires Cl- which enters the compartment via Gef1p. Therefore, copper and iron are limiting for growth at alkaline pH. Arr4p antagonizes the function of Gef1p (Metz et al., 2006). Thus, Arr4p is a negative regulator of Gef1p which binds directly to the C-terminus of the latter. GET3 (Arr4) thus inhibits Cl- transport via Gef1p (TC #1.A.11.1.1).
The GET complex is composed of the homodimeric Get3 ATPase and its heterooligomeric receptor, Get1/2. During insertion, the Get3 dimer shuttles between open and closed conformational states, coupled with ATP hydrolysis and the binding/release of TA proteins. Kubota et al. (2012) reported crystal structures of ADP-bound Get3 in complex with the cytoplasmic domain of Get1 (Get1CD) in open and semi-open conformations at 3.0- and 4.5-Å resolutions, respectively. Their structures and biochemical data suggest that Get1 uses two interfaces to stabilize the open dimer conformation of Get3. They propose that one interface is sufficient for binding of Get1 by Get3, while the second interface stabilizes the open dimer conformation of Get3. Evidence supports the conclusion that Get1/2 functions as the insertase directly (Kubota et al. 2012; Wang et al. 2014).
Entry of newly synthesized TA proteins into the GET pathway in Saccharomyces cerevisiae begins with efficient TMS capture by Sgt2 (a small glutamine-rich tetratricopeptide repeat-containing protein) (Denic 2012). This chaperone shields the TMS after it is released from the ribosome to prevent TA protein aggregation in the cytosol or mistargeting to mitochondria. Sgt2 is in a complex with Get4 and Get5, two pathway components that facilitate TA protein transfer from Sgt2 to Get3, a dimeric/tetrameric ATPase that is the ER membrane targeting factor of the GET pathway. This is achieved, first, when ATP stimulates binding of Get3 to Get4, and this increases the local concentration of Get3 near the TA protein because of the Get4-Get5-Sgt2 bridge. Second, Get4 increases the intrinsic rate of Get3-TA protein complex formation, most likely by making Get3 receptive for TMS binding. ATP binding converts Get3 from an open to a semi-closed state; ATP hydrolysis fully closes the Get3 conformation, creating a composite, hydrophobic groove that cradles the TMS. Tail anchors are sandwiched inside the dimeric Get3, which has a head-to-head arrangement of hydrophobic grooves (Denic 2012).
The structure of the Sgt2/Get5 complex is known (Simon et al. 2013) as is that of Get3 bound to different TA proteins which revealed the α-helical TMS occupying the hydrophobic groove that spans the Get3 homodimer (Mateja et al. 2015). The heterotetrameric Get4/Get5 complex (Get4/5), tethers the co-chaperone Sgt2 to the targeting factor, the Get3 ATPase. Crystal structures of the Get3·Get4/5 complex have also been solved (Gristick et al. 2015). In plants, an RK/ST C-terminal motif is required for targeting of OEP7.2 and a subset of other Arabidopsis tail-anchored proteins to the plastid outer envelope membrane (Teresinski et al. 2018). Two cytochrome b5 forms, b5-ER and b5-RR in animals, which differ only in the charge of the C-terminal region, target the endoplasmic reticulum (ER) or the mitochondrial outer membrane (MOM), respectively. Figueiredo Costa et al. 2018 demonstrated that the MOM is the preferred destination of spontaneously inserting TA proteins, regardless of their C-terminal charge, and revealed a novel, substrate-specific ER-targeting pathway.
The generalized reaction for the C-terminal tail-anchored membrane insertion complex is:
TA protein (cytoplasm) + ATP TA protein (membrane inserted) +ADP + Pi