Systematic and comprehensive analysis of major localizations of alpha-dystroglycan-specific modifying enzymes

Dystroglycan (DG) binds to extracellular matrix via its O-glycans, which are sequentially modified in a specific order by DG-specific enzymes: POMGNT2, B3GalNT2, and POMK in the endoplasmic reticulum (ER), followed by FKTN, FKRP, TMEM5, B4GAT1 and LARGE1 in the Golgi apparatus. However, there have been no comprehensive and systematic studies on the major localization of these enzymes. Here, we expressed fluorescent fusion proteins of DG-specific modifying enzymes under the control of short CMV promoter and observed their primary localization using the latest microscopy along with localization markers: mEGFP-KDEL for the ER, GM130 and GRASP55 for the cis-/medial-Golgi, and TGN46 and GCC1 for the trans-Golgi network. As a result, POMGNT2 and B3GalNT2 were localized to the ER as expected, but POMK was localized predominantly to the cis-/medial-Golgi showing co-localization with GRASP55. FKTN, FKRP and TMEM5 were partially co-localized with both cis-/medial- and trans-Golgi network markers. Though B4GAT1 did not co-localize with GM130 or TGN46, it co-localized with GCC1 another trans-Golgi network marker, indicating Golgi subcompartmentalization. LARGE1, the final glycosyltransferase involved in the modification of DG's O-glycan, was localized in the cis-/medial-Golgi, but did not overlap with trans-Golgi network markers. An EndoH sensitivity assay demonstrated that DG-specific enzymes interacting with DG were localized in the early secretory pathway. Our results reveal that POMK and B4GAT1 function at locations distinct from their major localization and support the conclusion that the modification of matriglycan on DG is completed at the cis-/medial-Golgi.