Responsible for addition of the terminal mannose residues to the outer chain of core N-linked polysaccharides and to O-linked mannotriose. Implicated in late Golgi modifications (By similarity). Protein modification; protein glycosylation. Expression is negatively regulated by RIM101. Expression is also increased in absence of SUR7, as well as CHK1 and NIK1. Belongs to the MNN1/MNT family.