Structural characterization of a eukaryotic chaperone—the ribosome-associated complex

The eukaryotic ribosome-associated complex (RAC) chaperone is poorly understood. Structural analyses now provide insight into the catalytic inactivity and possible functions of the Ssz1 subunit and reveal that RAC interacts with the ribosome via the Zuo1 subunit. RAC crouches over the ribosomal exit tunnel, where its conformation may be controlled by the ribosomal expansion segment ES27. Ribosome-associated chaperones act in early folding events during protein synthesis. Structural information is available for prokaryotic chaperones (such as trigger factor), but structural understanding of these processes in eukaryotes lags far behind. Here we present structural analyses of the eukaryotic ribosome-associated complex (RAC) from Saccharomyces cerevisiae and Chaetomium thermophilum , consisting of heat-shock protein 70 (Hsp70) Ssz1 and the Hsp40 Zuo1. RAC is an elongated complex that crouches over the ribosomal tunnel exit and seems to be stabilized in a distinct conformation by expansion segment ES27. A unique α-helical domain in Zuo1 mediates ribosome interaction of RAC near the ribosomal proteins L22e and L31e and ribosomal RNA helix H59. The crystal structure of the Ssz1 ATPase domain bound to ATP-Mg 2+ explains its catalytic inactivity and suggests that Ssz1 may act before the RAC-associated chaperone Ssb. Our study offers insights into the interplay between RAC, the ER membrane–integrated Hsp40-type protein ERj1 and the signal-recognition particle.