Fig. 4.

Hsp70 in protein translocation on the cytosolic side of the membrane. a, b Top: Nascent polypeptides (black line) travel through the tunnel of the ribosome (blue), before exiting into the cytosol. a Many polypeptides destined for the endoplasmic reticulum (ER) are synthesized with a targeting sequence at or near the N-terminus (yellow line segment). Left: Co-translational translocation. Signal recognition particle (SRP) binds targeting sequence (yellow), halting translation and targeting the nascent chain to SRP receptor (not shown), then to SEC61 translocon, at which point translation resumes. Right: Post-translational translocation. Nascent polypeptides with signal sequences not recognized efficiently by SRP are bound by Hsp70 Ssb (green, “B”) that is associated with ribosomes at the tunnel exit and/or soluble Hsp70 Ssa (gray, “A”). The Ssa C-terminal EEVD tetrapeptide is in red. Ssa and Ssb target nascent chains to SEC61 by binding to Sec72, a component of the Sec62/63 complex (Sec62/63, dark gray). Ssa interacts via its C-terminal EEVD tetrapeptide, and Ssb via its nucleotide binding domain. Not shown: J-proteins needed for Hsp70 binding to polypeptide substrate to facilitate ATP hydrolysis and NEF for exchange of nucleotide and thus release of substrate from Hsp70. b Mitochondria: Tom20 (pink cylinder) and Tom70 (purple cylinder), components of the TOM complex embedded in the outer membrane, are receptors for proteins destined for the inner membrane and matrix. Left: Proteins that bind Tom20 typically have an N-terminal, cleavable targeting sequence (cyan line segment). Right: Tom70 targeting sequences (orange line segment) are typically in the protein’s interior. Tom70 also binds the EEVD of Ssa type Hsp70s, helping to target these polypeptides to the TOM translocon