Transfer of mannose from mannosyl retinyl phosphate to protein

Abstract

Upon incubation of [14C]mannose-labeled mannosyl retinyl phosphate with a membrane fraction from rat liver, mannose was transferred to an endogenous acceptor precipitable withchloroform/methanol to the extent of about 7%. The reaction proceeded linearly with time for 120 min at a pH optimum of about 7.0. The acceptor thus labeled with mannose could be solubilized by sodium dodecyl sulfate/mercaptoethanol. More than half of this acceptor appeared in the void volume of a Sephadex G-100 column. When it was digested with Pronase, a substantial proportion of it appeared between the void and bed volumes of a Sephadex G-100 column, thus indicating that it was a glycopeptide. In high-voltage paper electrophoresis, this glycopeptide moved to the cathode at low pH and to the anode at highpH. When digested with highly purified jack bean alpha-mannosidase, the glycopeptide released almost 50% of its radioactivity as mannose. That this transfer of mannose to glycoprotein from mannosyl retinyl phosphate does not take place via dolichyl mannosyl phosphate was shown by the fact that it is Mn2+ and Mg2+ independent, it is not inhibited by the presence of a 10-fold molar excess of nonradioactive GDP-mannose, and neither 14C-labeled dolichyl mannosyl phosphate nor 14-C labeled lipid pyrophosphoryl oligosaccharide could be detected during the incubation.