The location of a second in vivo phosphorylation site in the A alpha-chain of human fibrinogen.

By quantitative phosphorus determination on the single chains of human fibrinogen it is demonstrated that the covalently bound phosphorus of adult and fetal fibrinogen is exclusively located in the A alpha chain. The A alpha-chain of fetal fibrinogen contains about twice as much phosphorus as the adult A alpha-chain in the well known position of Ser 3 of fibrino-peptide A as well as in a hitherto unknown second position on the A alpha-chain. By consecutive cleavage of the A alpha-chains of fetal and adult fibrinogen with cyanogen bromide, trypsin, and chymotrypsin, separation of the resulting peptide mixtures and analysis for phosphorylated amino acids, this second phosphorylation site could be traced to Ser 345 of the A alpha-chain. There is only one sequence homology between the two now known in vivo phosphorylation sites of human fibrinogen, namely that the second amino acid to the carboxyl side of the phosphorylated Ser is Glu. The sequence specificity of the up to now unidentified protein kinase phosphorylating fibrinogen allows it to be classified as a member of the group of type-2 casein kinases or casein kinases TS.