| Summary: | Functional characterization of the N‐glycans in lysosomal alpha‐mannosidase revealed a different sorting mechanism in the brain compared to peripheral organs. Ole K. Greiner‐Tollersrud, Christophe Flahaut, Jean C. Michalski, G. Evjen, Hilde M.F. Stensland, Karl E. Eilertsen, G. Hansen: VBRG, IMB, University of Tromso, 9037 Tromso, Norway Lysosomal alpha‐mannosidase (LAMAN) is a homodimer of 250 kDa, which is sorted to the lysosomes via the mannose 6‐P dependent pathway. N‐glycans linked to specific sites or regions of the folded LAMAN surface exerted multiple roles: 1) ER to Golgi transport of nascent LAMAN which requires an N‐glycan linked to a small region including positions 310 and 367; 2) Trafficking from Golgi to lysosomes via the mannose 6‐P dependent pathway which requires an N‐glycan linked to an accessible surface region of 40 Å in diameter, including positions 310, 367, 766 and 930; 3) Conformational stability in the lysosomes exerted by a high mannose structure linked to N497 within the dimer cleft; 4) Protection from lysosomal proteolyses by critically positioned N‐glycosylation sites in some species The surface region to which mannose‐phosphorylated glycans were attached, the “footstep” of GlcNAc 1‐phosphotransferase, included only one of the two LAMAN monomers, since to each of the N310, N367, N766 and N930‐sites there were linked two glycan types: A) GlcNAc2Man5‐7‐structures which are likely lysosomally processed forms of mannose 6‐phosphorylated glycans and B) Fucosylated paucimannosidic/bisected glycans which are processed forms of complex type glycans. The N‐glycans linked to the accessible N‐glycosylation sites outside this “footstep” were processed to endo H‐resistant complex glycans in bovine liver, kidney and lung. In bovine brain, however, these N‐glycans remained mainly endo H‐sensitive, and likely processed into hybrid type structures. Thus, brain LAMAN may be subjected to a unique trafficking diversion during transit through the Golgi complex.
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