RAG, a stable ribosome-associated complex in yeast formed by the DnaK-DnaJhomologs Ssz1p and zuotin

The yeast cytosol contains multiple homologs of the DnaK and DnaJ chaperone family. Our current understanding of which homologs functionally interact is incomplete. Zuotin is a DnaJ homolog bound to the yeast ribosome. We hav e now identified the DnaK homolog Ssz1p/Pdr13p as zuotin's partner chaperon e. Zuotin and Ssz1p form a ribosome-associated complex (RAC) that is bound to the ribosome via the zuotin subunit. RAC is unique among the eukaryotic DnaK-DnaJ systems, as the 1:1 complex is stable, even in the presence of AT P or ADP. In vitro, RAC stimulates the translocation of a rihosome-bound mi tochondrial precursor protein into mitochondria, providing evidence for its chaperone-like effect on nascent chains. In agreement with the existence o f a functional complex, deletion of each RAC subunit resulted in a similar phenotype in vivo. However, overexpression of zuotin partly rescued the gro wth defect of the Delta ssz1 strain, whereas overexpression of Ssz1p did no t affect the Delta zuo1 strain, suggesting a pivotal function for the DnaJ homolog.