Abstract
Membrane proteins are aggregation-prone in aqueous environments, and their biogenesis poses acute challenges to cellular protein homeostasis. How the chaperone network effectively protects integral membrane proteins during their post-translational targeting is not well understood. Here, biochemical reconstitutions showed that the yeast cytosolic Hsp70 is responsible for capturing newly synthesized tail-anchored membrane proteins (TAs) in the soluble form. Moreover, direct interaction of Hsp70 with the cochaperone Sgt2 initiates a sequential series of TA relays to the dedicated TA targeting factor Get3. In contrast to direct loading of TAs to downstream chaperones, stepwise substrate loading via Hsp70 maintains the solubility and targeting competence of TAs, ensuring their efficient delivery to the endoplasmic reticulum (ER). Inactivation of cytosolic Hsp70 severely impairs TA translocation in vivo Our results demonstrate a new role of cytosolic Hsp70 in directly assisting the targeting of an essential class of integral membrane proteins and provide a paradigm for how "substrate funneling" through a chaperone cascade preserves the conformational quality of nascent membrane proteins during their biogenesis.