Xenopus fibrinogen synthesis and secretion. Analysis of precursor polypeptides and their post-translational modification.

Xenopus fibrinogen is distinct from that of mammals in that the B beta subunit of the secreted protein has a higher apparent molecular weight than either the A alpha or the gamma subunit. A precursor polypeptide for each subunit was identified among the products translated in vitro from liver mRNA. Pre-A alpha is larger than pre-B beta and pre-gamma is the smallest of the three. Purified liver parenchymal cells cultured in the presence of tunicamycin secrete fibrinogen polypeptides which lack carbohydrate moieties. Our analysis of these forms of Xenopus fibrinogen demonstrated the presence of a signal peptide on each of the precursor polypeptides, the loss of a COOH-terminal peptide from the pre-A alpha chain, and the presence of one carbohydrate moiety on the mature gamma chain and two carbohydrate moieties on the mature B beta chain. The B fibrinopeptide on the NH2-terminus of the B beta chain is unusually large. The number of amino acids in this peptide is approximately twice that characteristic of mammalian B fibrinopeptides and it is glycosylated. On the basis of these data, together with information in the literature, we propose that when vertebrates first arose the B fibrinopeptide was a large glycosylated peptide. It retained this basic structure during the evolution of all subsequent vertebrates, with the exception of mammals. In contrast, the A fibrinopeptide increased in length early in vertebrate evolution. The alpha portion of the A alpha subunit of fibrinogen appears to have originally been a polypeptide about the same length as the beta chain. Concomitant with the evolution of mammals, the alpha polypeptide significantly increased in length.