Mouse osteopontin expressed in E. coli exhibits autophosphorylating activity of tyrosine residues

Osteopontin is a secreted glycosylated phosphoprotein found in various normal and transformed tissues. Mouse osteopontin expressed in bacteria has been found to autophosphorylate in vitro using ATP or GTP as phosphoryl donors. The reaction does not occur using inorganic orthophosphate as the donor. Only tyrosine residues are phosphorylated. Neither serine nor threonine residues, both of which are found phosphorylated in osteopontin extracted from bone, is autophosphorylated in vitro. The autophosphorylation of tyrosine residues by a secreted protein such as osteopontin may provide additional insight into its biological functions.