Identification of the Phosphorylation Sites of the Murine Small Heat Shock Protein Hsp25

Native phosphorylated mouse small heat shock protein hsp25 from Ehrlich ascites tumor cells was isolated and the in vivo phosphorylation sites of the protein were determined. Furthermore, native hsp25 was phosphorylated by the endogenous kinase(s) in a cell-free system as well as recombinant hsp25 was phosphorylated in vitro by protein kinase C and catalytic subunit of cAMP-dependent protein kinase. The two major phosphorylation sites of native and recombinant hsp25 were determined as Ser-15 and Ser-86. There are no differences in the hsp25 phosphorylation sites phosphorylated by the protein kinase C, the catalytic subunit of cAMP-dependent protein kinase and the unknown intracellular kinase(s). The serine residues identified exist in all known small mammalian stress proteins and are located in the conserved kinase recognition sequence Arg-X-X-Ser.