Functions of the α, β, and γ Subunits of UDP-GlcNAc:Lysosomal Enzyme N-Acetylglucosamine-1-phosphotransferase

Abstract

UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase is an alpha(2)beta(2)gamma(2) hexamer that mediates the first step in the synthesis of the mannose 6-phosphate recognition marker on lysosomal acid hydrolases. Using a multifaceted approach, including analysis of acid hydrolase phosphorylation in mice and fibroblasts lacking the gamma subunit along with kinetic studies of recombinant alpha(2)beta(2)gamma(2) and alpha(2)beta(2) forms of the transferase, we have explored the function of the alpha/beta and gamma subunits. The findings demonstrate that the alpha/beta subunits recognize the protein determinant of acid hydrolases in addition to mediating the catalytic function of the transferase. In mouse brain, the alpha/beta subunits phosphorylate about one-third of the acid hydrolases at close to wild-type levels but require the gamma subunit for optimal phosphorylation of the rest of the acid hydrolases. In addition to enhancing the activity of the alpha/beta subunits toward a subset of the acid hydrolases, the gamma subunit facilitates the addition of the second GlcNAc-P to high mannose oligosaccharides of these substrates. We postulate that the mannose 6-phosphate receptor homology domain of the gamma subunit binds and presents the high mannose glycans of the acceptor to the alpha/beta catalytic site in a favorable manner.