Evidence for the involvement of a 35-kDa membrane protein in the synthesis of glucosylphosphoryldolichol.

Photoactivatable (beta-32P)5-azidoUDPGlc binds to two proteins in rat liver microsomes. As determined by SDS-PAGE electrophoresis, the molecular masses of the 32P-labeled proteins were found to be 62- and 35-kDa. Binding of the photoprobe to both proteins was inhibited by addition of unlabeled UDPGlc. Labeling of the higher molecular weight protein occurred in the absence of photoactivation. In contrast, formation of the 32P-labeled 35-kDa protein was dependent on exposure of the membranes to UV light (250 nm). Moreover, labeling of the 35-kDa protein required the intact sugar nucleotide and divalent cations and was affected by the level of the endogenous and exogenous dolichylphosphate. All of these results are consistent with the possibility that the 35-kDa membrane protein is a component of glucosylphosphoryldolichol synthase.