A Chaperone Lid Ensures Efficient and Privileged Client Transfer during Tail-Anchored Protein Targeting.

Molecular chaperones play key roles in maintaining cellular proteostasis. In addition to preventing client aggregation, chaperones often relay substrates within a network while preventing off-pathway chaperones from accessing the substrate. Here we show that a conserved lid motif lining the substrate-binding groove of the Get3 ATPase enables these important functions during the targeted delivery of tail-anchored membrane proteins (TAs) to the endoplasmic reticulum. The lid prevents promiscuous TA handoff to off-pathway chaperones, and more importantly, it cooperates with the Get4/5 scaffolding complex to enable rapid and privileged TA transfer from the upstream co-chaperone Sgt2 to Get3. These findings provide a molecular mechanism by which chaperones maintain the pathway specificity of client proteins in the crowded cytosolic environment.