title: Evolution of alpha 2-macroglobulin. The structure of a protein homologous with human alpha 2-macroglobulin from plaice (Pleuronectes platessa L.) plasma.
abstract: The plaice (Pleuronectes platessa L.) papain-binding protein previously demonstrated to be homologous with human alpha(2)-macroglobulin, and designated plaice alpha(2)-macroglobulin homologue or alphaMh, was shown to be a glycoprotein of s(20,w) 11.86S. In polyacrylamide-gel pore-limit electrophoresis under non-denaturing conditions plaice alphaMh migrated to the same position as half-molecules of human alpha(2)-macroglobulin, and treatment with methylamine or a proteinase caused no change in its electrophoretic properties. Either denaturation in urea (4m) or mild reduction by dithiothreitol (1mm) partially dissociated plaice alphaMh into half-molecules. Denaturation with reduction further dissociated the protein into quarter-subunits. In sodium dodecyl sulphate/polyacrylamide-gel electrophoresis under reducing conditions plaice alphaMh dissociated into subunits of M(r) 105000 (I) and 90000 (II). Approximately equal amounts of each subunit were formed, and peptide ;mapping' showed subunits I and II to be distinct polypeptide chains. Under alkaline denaturing conditions, a proportion of the I chains of alphaMh were cleaved into fragments of M(r) about 60000 and 40000. This cleavage was favoured by reducing conditions and prevented by prior inactivation of the alphaMh with methylamine. [(14)C]Methylamine allowed to react with alphaMh became covalently linked to subunit I. These properties suggested the existence of an autolytic site on subunit I analogous to the autolytic site of human alpha(2)-macroglobulin. Reaction of alphaMh with a proteinase resulted in cleavage of a fragment of M(r) 10000-15000 from subunit I. A proportion of the proteinase molecules trapped by alphaMh became covalently linked to the inhibitor. A scheme is proposed for the evolution of human alpha(2)-macroglobulin and plaice alphaMh from a common ancestral protein, which may also have been an ancestor of complement components C3 and C4.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D002241:Carbohydrates / Q000032:analysis; D002352:Carrier Proteins / Q000032:analysis; D004591:Electrophoresis, Polyacrylamide Gel; D010450:Endopeptidases / Q000494:pharmacology; D005399:Fishes / Q000097:blood*; D006801:Humans; D010206:Papain / Q000378:metabolism; D010446:Peptide Fragments / Q000032:analysis; D011487:Protein Conformation; D011489:Protein Denaturation; D000511:alpha-Macroglobulins / Q000032:analysis*
chemical_list: D002241:Carbohydrates; D002352:Carrier Proteins; D010446:Peptide Fragments; D000511:alpha-Macroglobulins; D010450:Endopeptidases; D010206:Papain