Involved in O-glycosylation of cell wall and secreted proteins. Transfers an alpha-D-mannosyl residue from GDP-mannose into lipid-linked oligosaccharide, forming an alpha-(1->2)-D-mannosyl-D-mannose linkage. Mainly responsible for the addition of the second mannose residue in an O-linked mannose pentamer. Can also substitute for MNT2 by adding the third mannose residue. Important for adherence to host surfaces and for virulence. Protein modification; protein glycosylation. Belongs to the glycosyltransferase 15 family.