ABSTRACT
Disruption of the O-mannosylation pathway involved in functional glycosylation of α-dystroglycan gives rise to congenital muscular dystrophies. Protein O-linked mannose ß-1,4-N-acetylglucosaminyltransferase 2 (POMGNT2) catalyzes the first step toward the functional matriglycan structure on α-dystroglycan that is responsible for binding extracellular matrix proteins and certain arenaviruses. Alternatively, protein O-linked mannose ß-1,2-N-acetylglucosaminyltransferase 1 (POMGNT1) catalyzes the first step toward other various glycan structures present on α-dystroglycan of unknown function. Here, we demonstrate that POMGNT1 is promiscuous for O-mannosylated peptides, whereas POMGNT2 displays significant primary amino acid selectivity near the site of O-mannosylation. We define a POMGNT2 acceptor motif, conserved among 59 vertebrate species, in α-dystroglycan that when engineered into a POMGNT1-only site is sufficient to convert the O-mannosylated peptide to a substrate for POMGNT2. Additionally, an acceptor glycopeptide is a less efficient substrate for POMGNT2 when two of the conserved amino acids are replaced. These findings begin to define the selectivity of POMGNT2 and suggest that this enzyme functions as a gatekeeper enzyme to prevent the vast majority of O-mannosylated sites on proteins from becoming modified with glycan structures functional for binding laminin globular domain-containing proteins.