α-Mannosidases involved in N-glycan processing show cell specificity and distinct subcompartmentalization within the Golgi apparatus of cells in the testis and epididymis

Abstract

The Golgi apparatus is enriched in specific enzymes involved in the maturation of carbohydrates of glycoproteins. Among them, α-mannosidases IA, IB and II are type II transmembrane Golgi-resident enzymes that remove mannose residues at different stages of N-glycan maturation. α-Mannosidases IA and IB trim Man9GlcNAc2 to Man5GlcNAc2, while α-mannosidase II acts after GlcNAc transferase I to remove two mannose residues from GlcNAcMan5GlcNAc2 to form GlcNAcMan3GlcNAc2 prior to extension into complex N-glycans by Golgi glycosyltransferases. The objective of this study is to examine the expression as well as the subcellular localization of these Golgi enzymes in the various cells of the male rat reproductive system. Our results show distinct celland region-specific expression of the three mannosidases examined. In the testis, only α-mannosidase IA and II were detectable in the Golgi apparatus of Sertoli and Leydig cells, and while α-mannosidase IB was present in the Golgi apparatus of all germ cells, only the Golgi apparatus of steps 1-7 spermatids was reactive for α-mannosidase IA. In the epididymis, principal cells were unreactive for α-mannosidase II, but they expressed α-mannosidase IB in the initial segment and caput regions, and α-mannosidase IA in the corpus and cauda regions. Clear cells expressed n-mannosidase II in all epididymal regions, and α-mannosidase IB only in the caput and corpus regions. Ultrastructurally, α-mannosidase IB was localized mainly over cis saccules, α-mannosidase IA was distributed mainly over trans saccules, and α-mannosidase II was localized mainly over medial saccules of the Golgi stack. Thus, the cell-specific expression and distinct Golgi subcompartmental localization suggest that these three α-mannosidases play different roles during N-glycan maturation.