CELL-SURFACE AND SERUM-PROTEIN PHOSPHORYLATION BY U-937 CELL ECTOPROTEIN KINASES

Incubation of intact U-937 cells with 0.1 mu M [gamma-P-32]ATP for 10 min resulted in Mg2+-independent radiolabelling of about 40 cell surfa ce proteins, some of which became more intensely labelled when cells w ere differentiated. Presence of 2 % newborn calf serum revealed major labelling of three serun proteins with molecular weights of 170, 85 an d 61 kDa. Out of several tested purified human serum proteins, complem ent factor Is (Cls) exhibited specific labelling of the 28 kDa subunit . This capacity of monocytic cell ectokinases to phosphorylate cell su rface and serum proteins may have significance for the regulation of i nteractions between these cells and their environment. In addition, ph osphorylation of components of the complement system suggest a possibl e new means of regulating the immune response through the action of ex tracellular kinases.