Identification of two 36-kD phosphoproteins associated with altered differentiation in retrovirus-transformed...

Balb/MK-2 cells, derived from mouse epidermal keratinocytes, require epidermal growth factor (EGF) for growth and undergo terminal differentiation when exposed to extracellular calcium levels greater than 1.0 mM. Transformation of these cells by a variety of acute transforming viruses abrogates the EGF requirement and blocks terminal differentiation. In general, oncogenes coding for tyrosine kinase oncoproteins abolish differentiation at an earlier step than those of the ras gene family. We therefore examined whether alterations in protein phosphorylation occur during differentiation of the Balb/MK-2 cells and two different viral transformants. Only one 36-kD phosphoprotein emerged whose phosphorylation consistently changed during epidermal differentiation. Modifications in the phosphorylation of a second 36-kD protein occurred in the virally transformed cell lines. Phosphoamino acid analysis of the proteins demonstrated only the presence of phosphoserine residues. These studies define changes in protein phosphorylation associated with the regulation of epidermal differentiation.