Mouse α-macroglobulin: structure, function and a molecular model

Mouse alpha-macroglobulin (M-AMG) is believed to be a functional homologue of human alpha 2-macroglobulin (h-alpha 2M). The subunit composition, the tryptic cleavage pattern before and after methylamine incorporation and the two-dimensional tryptic-peptide mapping, however, indicate that these two proteins are structurally distinct. M-AMG is composed of two major types of polypeptides (Mr 163,000 and 35,000) together with a minor polypeptide (Mr 185,000), whereas h-alpha 2M has only one type of polypeptide (Mr 185,000). After incorporation of methylamine, there is no change in the normal tryptic-cleavage pattern of M-AMG; however, tryptic cleavage of h-alpha 2M is severely retarded [Hudson & Koo (1982) Biochim. Biophys. Acta 704, 290-303]. The N-terminal sequence of the 163,000-Mr polypeptide of M-AMG shows sequence homology with the N-terminal sequence of h-alpha 2M. The amino acid compositions of M-AMG and its two major polypeptide chains are compared. Thermal fragmentation studies show that the 163,000-Mr polypeptide is broken down into 125,000-Mr and 29,000-Mr fragments. Trypsin-binding studies show that M-AMG can bind two molecules of trypsin/molecule. Inactivations of the trypsin-binding property of M-AMG and h-alpha 2M with methylamine show similar kinetics of inhibition at 4 degrees C. A structural model of M-AMG is proposed, based on accumulated data.