POSSIBLE ROLE FOR SERINE THREONINE PHOSPHORYLATION IN THE REGULATION OF THE HETEROPROTEIN COMPLEX BETWEEN THE HSP90 STRESS PROTEIN AND THE PP60(V-SRC) TYROSINE KINASE/

The abundant, cytoplasmic 90-kDa heat-shock protein associates transie ntly with the Rous sarcoma virus oncogenic protein tyrosine kinase, pp 60(v-src), directs its cellular trafficking and negatively regulates i ts kinase activity, Here we report that the serine/threonine phosphata se inhibitor, okadaic acid, destabilized the heat-shock protein 90-pp6 0(v-src) chaperone complex in v-src-transfected cells. Concomitant wit h complex destabilization by okadaic acid, phosphoserine was doubled a nd phosphothreonine was increased 20-fold in the heat-shock protein 90 . Although phosphorylation of the total pool of immunoprecipitable pp6 0(v-src) was unchanged, okadaic acid slightly increased phosphoserine and phosphothreonine levels specifically in pp60(v-src) bound to heat- shock protein 90. The low level of tyrosine phosphorylation in the pp6 0(v-src) complexed with heat-shock protein 90 was further decreased by okadaic acid. Interestingly, okadaic acid-stabilized hyperphosphoryla tion of the heat-shock protein 90-pp60(v-src) complex lowered the leve l of pp60(v-src) in cell membranes, the functional location for pp60(v -src). We suggest that serine/threonine phosphorylation of heat-shock protein 90 and/or pp60(v-src) functions as a regulatory molecular trig ger to release pp60(v-src) from the chaperone complex at the inner sur face of cell membranes.