The structure of a transmembrane protein sorting complex

The biogenesis of membrane proteins is an essential process in biology. It requires the protection of hydrophobic transmembrane domains from aggregation in the cytosol as well as targeting to the proper membrane. Tail-anchored (TA) proteins have a single transmembrane helix near their carboxyl termini and require a post-translational mechanism for targeting and insertion. In yeast, the Guided Entry of Tail-anchored proteins (GET) pathway delivers TA proteins to the endoplasmic reticulum (ER). A sorting complex comprising Get4, Get5, and Sgt2 load ER destined TA proteins onto the targeting factor Get3. X-ray crystallography, solution NMR, and small angle X-ray scattering were used to characterize this assembly. Get4 and Get5 form an extended adapter complex. Get4 maintains Get3 in a state competent to receive TA proteins. The N-terminus of Get5 tightly binds Get4, while the C-terminus of Get5 is a homodimerization domain, resulting in a heterotetrameric assembly. A ubiquitin-like domain within Get5 binds the heat-shock protein (HSP) co-chaperone Sgt2, providing a physical link between ER destined TA protein targeting and protein folding pathways. Sgt2 is also an extended homodimeric complex, and can directly bind four major classes of HSPs. The Get4/Get5/Sgt2 sorting complex is multivalent, flexible and the binding of individual components is transient. These results build a model for post-translational protein targeting in eukaryotes that is distinct from other pathways.