Get5 Carboxyl-terminal Domain Is a Novel Dimerization Motif That Tethers an Extended Get4/Get5 Complex

Tail-anchored trans-membrane proteins are targeted to membranes post-translationally. The proteins Get4 and Get5 form an obligate complex that catalyzes the transfer of tail-anchored proteins destined to the endoplasmic reticulum from Sgt2 to the cytosolic targeting factor Get3. Get5 forms a homodimer mediated by its carboxyl domain. We show here that a conserved motif exists within the carboxyl domain. A high resolution crystal structure and solution NMR structures of this motif reveal a novel and stable helical dimerization domain. We additionally determined a solution NMR structure of a divergent fungal homolog, and comparison of these structures allows annotation of specific stabilizing interactions. Using solution x-ray scattering and the structures of all folded domains, we present a model of the full-length Get4/Get5 complex. The Get4/Get5 protein complex is a homodimer mediated by the Get5 carboxyl domain. The Get5 homodimerization motif forms a structurally conserved helical domain allowing Get4/Get5 to adopt an extended solution conformation. Get5 homodimerization is mediated by a 35-residue sequence stabilized by a few conserved hydrophobic interactions. The Get5 carboxyl domain contains a novel example of a stable dimerization motif.