Exoglycosidase that cleaves the single beta-linked mannose residue from the non-reducing end of all N-linked glycoprotein oligosaccharides. Hydrolysis of terminal, non-reducing beta-D-mannose residues in beta-D-mannosides. Optimum pH is 4.5. Glycan metabolism; N-glycan degradation. Monomer. Highest level in liver, high levels in lung, testis, skin and spleen, moderate level in thymus. Activity found in plasma, kidney, liver, spleen, pancreas, brain, testis, epididymis, heart, lung and skeletal muscle. Mice are born at the expected Mendelian rate and display no visible phenotype for at least one year. However, they display vacuolation in the central nervous system and accumulation of disaccharides in brain and epididymis, detectable already at four weeks after birth. Belongs to the glycosyl hydrolase 2 family.