A 16-KDA PROTEIN FUNCTIONS AS A NEW REGULATORY PROTEIN FOR HSC70 MOLECULAR CHAPERONE AND IS IDENTIFIED AS A MEMBER OF THE NM23 NUCLEOSIDE DIPHOSPHATE KINASE FAMILY/

Cytoplasmic Hsc70 is a multifunctional molecular chaperone. It is hypo thesized that accessory proteins are used to specify the diverse chape rone activities of Hsc70. A 16-kDa cytosolic protein (p16) co-purified with Hsc70 obtained from a fish hepatocyte cell line, PLHC-1. Hsc70 a lso co-immunoprecipitated with p16 from PLHC-1 cells and fish liver. p 16 was identified as a member of the Nm23/nucleoside diphosphate (NDP) kinase family based on its amino acid sequence similarity, NDP kinase activity, and recognition by anti-human NDP kinase-A antibody. This a ntibody also co-immunoprecipitated Hsc70 and NDP kinase from human Hep G2 cells. p16 monomerized Hsc70 and released Hsc70 from pigeon cytochr ome c peptide (Pc) but not from FYQLALT, a peptide specifically design ed for high affinity binding. Therefore, p16 may modulate Hsc70 functi on by maintaining Hsc70 in a monomeric state and by dissociating unfol ded proteins from Hsc70 either through protein-protein interactions or by supplying ATP indirectly through phosphate transfer. p16 did not a ffect basal or unfolded protein-stimulated ATPase activity of bovine b rain Hsc70 using in vitro assays. Interestingly, bovine liver MDP kina se did not dissociate the Hsc70 Pe complex. In addition, two nonconser vative amino acid subsitutions were found near the amino terminus of p 16. Therefore, p16 may be a unique Nm23/NDP kinase that functions as a n accessory protein for cytosolic Hsc70 in eukaryotes.