Cell surface and serum protein phosphorylation by U‐937 cell ectoprotein kinases

Abstract

Incubation of intact U-937 cells with 0.1 microM [gamma-32P]ATP for 10 min resulted in Mg(2+)-independent radiolabelling of about 40 cell surface proteins, some of which became more intensely labelled when cells were differentiated. Presence of 2% newborn calf serum revealed major labelling of three serum proteins with molecular weights of 170, 85 and 61 kDa. Out of several tested purified human serum proteins, complement factor 1s (C1s) exhibited specific labelling of the 28 kDa subunit. This capacity of monocytic cell ectokinases to phosphorylate cell surface and serum proteins may have significance for the regulation of interactions between these cells and their environment. In addition, phosphorylation of components of the complement system suggest a possible new means of regulating the immune response through the action of extracellular kinases.