124-v-Mos phosphorylates α- and β-casein in vitro. Mos kinase assays, in the presence of α- and β-casein, were resolved using 10% SDS-PAGE; the Coomassie stained protein gel shown in 3A, right panel and the corresponding autoradiograph on the left panel. Arrowheads indicate the position of 124-v-Mos, α- and β-casein and the antibody. Using two control immunoprecipitates of Sf9 cells expressing the synthetic kinase-inactive constructs, 124-v-MosK121R or PKCγK380R, Mos-specific β-casein phosphorylation was demonstrated in 3B and 3C: Mos kinase assays were blotted on nylon-membrane, the phospho-β-casein bands (B, arrowhead) excised and 32P-Cerenkov counts recorded (B). Alternatively, the excised phospho-β-casein bands were digested with trypsin and electrophoresed using 16% SDS-PAGE (C). The arrowhead in 3C indicates the tryptic β-casein peptide phosphorylated by wild-type 124-v-Mos only. Further, two-dimensional phosphoamino acid analyses of 124-v-Mos phosphorylated α-casein (D, left panel) and β-casein (D, right panel) were completed, the arrowheads indicating the origins of sample application.