Ribosome-bound, Hsp70-type chaperone that assists in the cotranslational folding of newly synthesized proteins in the cytosol. Stimulates folding by interacting with nascent chains, binding to short, largely hydrophobic sequences exposed by unfolded proteins, thereby stabilizing longer, more slowly translated, and aggregation-prone nascent polypeptides and domains that cannot fold stably until fully synthesized. The Hsp70-protein substrate interaction depends on ATP-binding and on allosteric regulation between the NBD and the SBD. The ATP-bound state is characterized by a fast exchange rate of substrate (low affinity state), while in the ADP-bound state exchange is much slower (high affinity state). During the Hsp70 cycle, the chaperone switches between the ATP-bound state (open conformation) and the ADP-bound state (closed conformation) by major conformational rearrangements involving mainly the lid domain. Ssb cooperates with a specific Hsp40/Hsp70 co-chaperone termed the ribosome-associated complex (RAC), which stimulates the ATPase activity of the ribosome-associated pool of Ssbs and switches it to the high affinity substrate binding state. Hsp110 chaperone SSE1 and FES1 act as nucleotide exchange factors that cause substrate release. ATP + H2O = ADP + H(+) + phosphate kcat is 0.95 min(-1) with ATP as substrate (at 2.5 mM potassium acetate) and 0.81 min(-1) with ATP as substrate (at 100 mM potassium acetate). Binds to ribosomes (PubMed:9670014, PubMed:1394434, PubMed:27917864). Binds close to the ribosomal tunnel exit via contacts with both ribosomal proteins RPL35, RPL39 and RPL19, and rRNA (PubMed:27882919). Directly interacts with nascent polypeptides. This interaction is dependent on the ribosome-associated complex (RAC) (PubMed:11929994, PubMed:23332755). Interacts with SSE1 (PubMed:16219770, PubMed:16221677, PubMed:23332755). Interacts with FES1 (PubMed:17132105). Interacts with NAP1 (PubMed:18086883). About 50% of the protein is associated with translating ribosomes, but sufficient Ssb exists in the cell for each ribosome to be associated with at least one Ssb molecule. Expression decreases after heat shock or during growth to stationary phase (PubMed:6761581, PubMed:2651414). Degraded during heat shock treatment (at protein level) (PubMed:7646503). Up-regulated upon carbon upshift and down-regulated upon amino acid limitation in an HSF1-dependent manner (PubMed:10322015). Present with 170000 molecules/cell in log phase SD medium. Belongs to the heat shock protein 70 family. Ssb-type Hsp70 subfamily.