PROTEIN-KINASE C-DEPENDENT IN-VIVO PHOSPHORYLATION OF PROUROKINASE LEADS TO THE FORMATION OF A RECEPTOR COMPETITIVE ANTAGONIST

We recently reported that in vivo phosphorylation of urokinase-type pl asminogen activator on Ser(138/303) prevents its catalytic independent ability to promote myelomonocytic cell adherence and motility, me now show that Ca2+ activated, phospholipid-dependent protein kinase C fro m rat brain phosphorylates in vitro a peptide corresponding to prourok inase residues 133-143 (DGKKPSSPPEE) and the full-length molecule on S er(138/139). The in vivo involvement of the protein kinase C isoenzyme family is supported by the finding that inhibition of kinase C activi ty prevents prourokinase phosphorylation on Ser(138/303) in A431 human carcinoma cells. Conversely, a short treatment of A431 cells with pho rbol myristate acetate increases the extent of phosphorylated prouroki nase and, concomitantly, affects its function; under these conditions, the capability of prourokinase to up-regulate U937 monocyte-like cell adherence is severely impaired, although receptor binding is unaltere d. By the aid of a ''phosphorylation-like'' variant (Se-138 to Glu) we show that modification of Ser(138) is sufficient to confer to prourok inase the antagonistic properties observed following in vivo stimulati on of protein kinase C activity. These observations provide the first evidence that protein kinase C directs the formation of a receptor com petitive antagonist by regulating the in vivo phosphorylation state of prourokinase.