“…Using exogenous ( Y~~P ) A T P as substrate it was apparent that the reaction took place at the outer surface of intact Ehrlich cells and resulted in phosphorylation of the protein-bound seryl-and threonyl-residues of the phosphate acceptor protein(s) (3,4,35,36). Endogenous protein kinases effecting phosphorylation of endogenous membrane proteins have since then been found in several tissues including brain membrane fragments and synaptic preparations (22,29,45,47,49), human red cell membranes (6,20,39,40,41), rat liver plasma membranes (3 1, 44) and mitochondria1 membranes (32), bovine anterior pituitary particulate fractions including plasma membranes (26,27,43), membrane preparations from bovine corpus luteum (7, 8) and hamster ovary cells (9), mouse mammary gland (30), renal medulla (15), cortex (17) and brush border (l), toad bladder (13, 14), canine heart (51), adrenal gland (48), myelin basic protein (lo), HeLa cells (24,' 33), muscle cells (5) and fat cells (11,21).…”