CHARACTERIZATION OF A 54-KILODALTON HUMAN PROTEIN-KINASE RECOGNIZED BY AN ANTISERUM RAISED AGAINST THE MYOTONIN KINASE

We characterized a 54-kDa human protein kinase recognized by an antise rum raised against the human myotonin protein kinase. This protein kin ase displays a serine/threonine kinase activity in the heart and a tyr osine kinase activity in the skeletal muscle. Both kinase activities w ere attributed to the same 54-kDa protein based on the identity of one -dimensional peptide maps. We showed that the tyrosine kinase activity observed in the skeletal muscle results from a phosphorylation of thi s protein kinase on tyrosine residues by a tyrosine kinase specificall y expressed in this tissue. The tyrosine dephosphorylation of the skel etal muscle 54-kDa protein kinase allowed it to phosphorylate with the highest activity the same peptide substrates as those phosphorylated by the human recombinant myotonin kinase. These results show that a mu scle-specific tyrosine phosphorylation event converts a serine/threoni ne kinase to a tyrosine kinase. They also suggest that the 54-kDa prot ein kinase is a member of the myotonin kinase family. (C) 1998 John Wi ley & Sons, Inc.