Effect of divalent metal ions on the synthesis of oligosaccharide side chains of Neurospora crassa glycoproteins

Neurospora crassa membrane preparations incorporated mannose from GDP-mannose-[¹⁴C] in the presence of Mg²⁺ into a polyprenol lipid and side chains of protein acceptor(s), which are labile on hydrolysis in weak base. The addition of Mn²⁺ to the reaction mixtures does not affect mannosyl lipid synthesis but it stimulates the transfer of mannose to larger oligosaccharide chains resistant to β-elimination and the transfer of a second mannosyl unit to form an O-glycosidically linked mannobiosyl side chain. Incubation of particulate preparations with polyprenol-mannose-[¹⁴C] in the presence of Mg²⁺ and Mn²⁺ also results in the transfer of a single mannose to the protein. When non-radioactive GDP-mannose is added to this reaction mixture, however, β-elimination yields mannobiose. The mannobiose is labeled in the reducing sugar only. These results indicate that the first mannose of this mannobiosyl side chain is transferred via a lipid intermediate, but the second mannose is transferred directly from GDP-mannose. In the presence of Mg²⁺ and Mn²⁺, mannose apparently is also transferred from polyprenol-mannose-[¹⁴C] to side chains which are resistant to hydrolysis.