The biosynthesis of oligosaccharide-lipids. Activation of mannosyltransferase II by specific phospholipids.

Mannosyltransferase II catalyzes transfer directly from GDP-mannose to an oligosaccharide-lipid resulting in the formation of the alpha-1,3-mannosyl linkage in Man alpha 1-3(Man alpha 1-6)Man beta-GlcNAc beta-GlcNac-P-P-lipid. The enzyme has been solubilized and purified 660-fold from rabbit liver microsomes (Jensen, J. W., and Schutzbach, J. S. (1981) J. Biol. Chem. 256, 12899-12904). Enzyme activity was reconstituted in the presence of specific phospholipids and evidence for the formation of an enzyme-phospholipid complex was obtained from kinetic studies, by the stabilization of the enzyme by phosphatidylethanolamine, and by co-sedimentation of the enzyme with phospholipid. Maximal activity was restored only when the enzyme was reconstituted in the presence of synthetic or naturally occurring phosphatidylethanolamine which contained unsaturated acyl chains. Investigation of the phospholipid specificity required for activation of the enzyme and results obtained with phospholipid vesicles of mixed composition suggest that the enzyme may have a requirement for phospholipids that can associate to form nonbilayer lipid structures in aqueous environments.