Induction of an unusual type of shared phosphorylation in human and avian cells by tumor-promoting phorbol esters or transformation.

Alterations in patterns of protein phosphorylation after exposure to phorbol esters were compared in chicken embryo fibroblasts and KD cells (a human fibroblast line) by two-dimensional gel electrophoresis. A substantial increase in phosphorylation was observed of a major, markedly acidic protein of pI = 4.5 in two-dimensional gels of each cell type. However, the apparent molecular weights of these phosphoproteins differed substantially in the two species with a molecular weight of 67,000 in chicken fibroblasts and one of 80,000 in human KD cells. Both phosphoproteins, termed 67K and 80K respectively, contained phosphoserine and a small amount of phosphothreonine, but no detectable level of phosphotyrosine. Tryptic and chymotryptic phosphopeptide maps of 67K were nearly identical to those of 80K. These results indicate that they are related molecules, even though considerably different in apparent molecular weight, and that the induction of phosphorylation of these closely related, major, acidic phosphoproteins by phorbol esters is conserved from avian to human cells. In chicken embryo fibroblasts infected by a temperature-sensitive mutant of Rous sarcoma virus, phosphorylation of 67K was found to be elevated at 36 degrees C (transformed phenotype) compared to 41.5 degrees C (normal). Although the function of these closely related 67K and 80K phosphoproteins is unknown, the elevated level of phosphorylation could be involved in some aspects of transformation, and the increase is mimicked by treatment with phorbol esters.