The dystroglycanopathies are a novel group of human muscular dystrophies due to mutations in known or putative glycosyltransferase enzymes. They share the common pathological feature of a hypoglycosylated form of alpha-dystroglycan, diminishing its ability to bind extracellular matrix ligands. The LARGE glycosyltransferase is mutated in both the myodystrophy mouse and congenital muscular dystrophy type 1D (MDC1D). We have transfected various cell lines with a variety of LARGE expression constructs in order to characterize their subcellular localization and effect on alpha-dystroglycan glycosylation. Wild-type LARGE co-localized with the Golgi marker GM130 and stimulated the production of highly glycosylated alpha-dystroglycan (hyperglycosylation). MDC1D mutants had no effect on alpha-dystroglycan glycosylation and failed to localize correctly, confirming their pathogenicity. The two predicted catalytic domains of LARGE contain three conserved DxD motifs. Systematically mutating each of these motifs to NNN resulted in the mislocalization of one construct, while all failed to have any effect on alpha-dystroglycan glycosylation. A construct lacking the transmembrane domain also failed to localize at the Golgi apparatus. These results indicate that LARGE needs to both physically interact with alpha-dystroglycan and function as a glycosyltransferase in order to stimulate alpha-dystroglycan hyperglycosylation. We have also cloned and overexpressed a homologue of LARGE, glycosyltransferase-like 1B (GYLTL1B). Like LARGE it localized to the Golgi apparatus and stimulated alpha-dystroglycan hyperglycosylation. These results suggest that GYLTL1B may be a candidate gene for muscular dystrophy and that its overexpression could compensate for the deficiency of both LARGE and other glycosyltransferases.