KINASE-ACTIVITIES OF C-MOS AND V-MOS PROTEINS - A SINGLE AMINO-ACID EXCHANGE IS RESPONSIBLE FOR CONSTITUTIVE ACTIVATION OF THE 124-V-MOS KINASE

The Mos protein kinase is a serine-/threonine-specific protein kinase with a crucial role in meiotic cell divisions in vertebrates. Several oncogenic derivatives of the c-Mos protein have been discovered in mur ine retroviruses. These proteins have acquired mutations and exhibit d ifferent degrees of protein kinase activity in vitro. In an attempt to understand the factors governing Mos protein kinase activity we have compared the kinase activities of the wild-type c-Mos protein and of t wo v-Mos proteins (strain HT1 and MSV124) after expression in insect c ells. Only the 124 v-Mos protein showed kinase activity in vitro as me asured by autophosphorylation, vimentin phosphorylation or by phosphor ylation and activation of MAP kinase kinase. By domain swapping and si te-directed mutagenesis we identified a single point mutation in the 1 24 v-Mos protein (Arg(145)-->Gly) which is responsible for its constit utive activity. This residue is located in the alpha-helix C of the ki nase domain close to the ATP binding fold and is conserved in all know n c-Mos proteins. Introduction of the corresponding mutation into HT1 v-Mos and into murine c-Mos activated bath proteins for autophosphoryl ation, vimentin phosphorylation and for signalling via MAP kinase kina se in vitro. We hypothesize that the Arg(145)-->Gly mutation found in 124 v-Mos mimicks a conformational change which might be an obligatory step in the activation of c-Mos in vivo.