Phosphorylation of keratin polypeptides

The phosphorylation of keratin polypeptides was examined in calf snout epidermis. When slices of epidermis were incubated in the medium containing ³²P_i, the radioactivity was incorporated into several proteins. The predominant phosphorylated proteins migrated in SDS-polyacrylamide gels with apparent molecular weight between 49000 and 69000 and coincided with keratin polypeptides. The extent of keratin phosphorylation was not altered in the presence of dibutyryl cyclic AMP or reagents which elevate intracellular cyclic AMP. When homogenates of epidermis were incubated with γ-³²P]ATP, keratin polypeptides were the predominant species phosphorylated as was also observed in epidermal slices. The presence of cyclic AMP or heat-stable inhibitor of cyclic AMP-dependent protein kinase in the reaction mixture did not affect the phosphorylation of keratin polypeptides, although the phosphorylation of exogenously-added histone was stimulated and inhibited, respectively, by these additions. Keratin polypeptides extracted from calf snout epidermis by 8 M urea were phosphorylated by incubation with γ-³²P]ATP and cyclic AMP-dependent protein kinase form calf snout epidermis or bovine heart. No proteins were phosphorylated without the addition of the enzymes. The presence of cyclic AMP in the reaction mixture stimulated the keratin phosphorylation, and further addition of heat-stable protein kinase inhibitor reduced this stimulation.