Phosphorylation of fibrinogen by casein kinase 1.

Casein kinase 1 phosphorylated human fibrinogen, in a reaction that did not use GTP as phosphoryl donor and was neither stimulated by cyclic AMP or Ca2+, nor inhibited by the cyclic AMP-dependent protein kinase inhibitor protein. Maximal incorporation averaged 4 mol of phosphate per mol of fibrinogen, most of it in the largest CNBr-fragment of the alpha-chain. Phosphoamino acid analysis revealed that phosphorylation occurred only at seryl residues. The phosphorylation of fibrinogen by casein kinase 1 was reverted by alkaline phosphatase.