Ultrastructure and composition of bovine conglutinin

Conglutinin binds in a Ca2+-dependent manner to the carbohydrate portion of zymosan and cell-bound iC3b (complement subcomponent C3b cleaved by Factor I in the presence of factor H) similarly to lectin-like proteins that participate in the clearance of plasma glycoproteins. This carbohydrate-binding protein has been found to include both collagenous and non-collagenous domains. Electron micrographs of bovine conglutinin are presented in which conglutinin appears as a tetramer of four 'lollipop' structures emanating from a central hub. The stem region, linking each head to the central hub, is quite stiff, whereas the hub-stem junction is a flexible hinge. From electron micrographs of a pepsin digest of conglutinin, the linkage region is identified as the collagenous portion of the macromolecule. Conglutinin is a multimer of a single polypeptide chain. From sedimentation equilibria of unreduced as compared with reduced and alkylated conglutinin, there are determined to be three disulphide-linked chains. These data, combined with information on the subunit polypeptide of conglutinin, suggest a model for conglutinin in which four disulphide-linked trimers are associated via the N-termini to form the intact macromolecule as viewed in the electron microscope. The ultrastructure of conglutinin appears ideally suited to its lectin-like function.