Glycosidase that specifically hydrolyzes the Man-beta-1,4-GlcNAc linkage in the trimannosyl core structure of N-glycans. Does not hydrolyzes pyridylamino derivatives sugar chains containing Man-alpha-1,3-Man-beta or Xylose-beta-1,2-Man-beta. Hydrolysis of the alpha-D-mannosyl-(1->6)-beta-D-mannosyl-(1->4)-N-acetyl-beta-D-glucosaminyl-(1->4)-N-acetyl-beta-D-glucosaminyl sequence of glycoprotein to alpha-D-mannosyl-(1->6)-D-mannose and N-acetyl-beta-D-glucosaminyl-(1->4)-N-acetyl-beta-D-glucosaminyl sequences. Optimum pH is 5.0. Heterotrimer of 31 kDa, 28 kDa and 42 kDa subunits. The mature enzyme is proteotically cleaved into 3 subunits of 31 kDa, 28 kDa and 42 kDa. Belongs to the glycosyl hydrolase 2 family.