Catalyzes the first committed step in the biosynthesis of complex N-glycans. It controls conversion of high mannose to complex N-glycans; the final hydrolytic step in the N-glycan maturation pathway. Converts GlcNAcMan(5)GlcNAc(2) (Man5Gn) into GlcNAcMan(3)GlcNAc(2) (MGn) by sequential removal of two alpha1,6- and alpha1,3-linked mannose residues from the alpha1,6-mannose branch of the substrate. To a lesser extent, also able to cleave beta1,2-xylosylated Man5Gn-glycopeptide (Man5GnX-GP) and pyridylaminated substrates Man5Gn-PA and Man5GnX-PA, but not active toward Man5-glycopeptide (PubMed:16460512, PubMed:21478158). Required for resistance to salt stress (PubMed:18408158). 2 H2O + N(4)-{beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->3)-[alpha-D-Man-(1->3)-[alpha-D-Man-(1->6)]-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] = 2 alpha-D-mannopyranose + N(4)-{beta-D-GlcNAc-(1->2)-alpha-D-Man-(1->3)-[alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-beta-D-GlcNAc}-L-asparaginyl-[protein] Binds 1 zinc ion per subunit. Not sensitive to EDTA and not significantly stimulated by cations such as Ca(2+), Co(2+), Mn(2+), Ni(2+) or Zn(2+) (PubMed:16460512). Inhibited by 1 mM Cu(2+) and by the class II alpha-mannosidase inhibitor swainsonine. Protein modification; protein glycosylation. Homodimer; disulfide-linked (By similarity). Interacts with GALT1 (PubMed:23400704). Glycosylated. Predominant presence of unprocessed hybrid N-glycans (PubMed:16460512). Reduced levels of glycoprotein N-glycans (PubMed:21478158). Increased sensitivity to salt stress (PubMed:18408158). Belongs to the glycosyl hydrolase 38 family. Truncated N-terminus.