Alpha-1,2-mannosyltransferase required for cell wall integrity. Responsible for addition of the first alpha-1,2-linked mannose to form the branches on the mannan backbone of oligosaccharides. Addition of alpha-1,2-mannose is required for stabilization of the alpha-1,6-mannose backbone and hence regulates mannan fibril length; and is important for both immune recognition and virulence. Protein modification; protein glycosylation. Leads to sensitivity to calcofluor white and SDS, as well as to thermosensitivity. Belongs to the MNN1/MNT family.