Effect of microinjected catalytic fragment of protein kinase C on morphological change in Swiss 3T3 cells

Although phorbol esters can enhance formation of an active, catalytic domain of protein kinase C (PKC) in intact cells, little is known about the actual importance of the proteolytic pathway in mediating cellular responses to the phorbol esters or other PKC activators. To explore this issue, we examined the effect of microinjected catalytic fragment of PKC on Swiss 3T3 cell morphology. In contrast to the dramatic, rapid response upon phorbol ester treatment, catalytic fragment microinjected in the presence of bovine serum albumin or normal goat immunoglobulin G as carrier protein had no effect. A morphological response similar but not identical to the effect of phorbol ester treatment was obtained, however, if catalytic fragment was microinjected in the presence of normal rabbit immunoglobulin G rather than the usual carrier proteins. The normal rabbit immunoglobulin by itself was inactive. Although the mechanism remains undefined, normal rabbit immunoglobulin but not other carrier proteins modulated PKC activity in vitro. We conclude that the generation of free catalytic fragment of PKC cannot account for the morphological response of Swiss 3T3 cells to the phorbol esters; secondary factors may, however, potentiate its action.