Alpha-1,2-mannosyltransferase, responsible for addition of the first alpha-1,2-linked mannose to form the branches on the mannan backbone of oligosaccharides. Protein modification; protein glycosylation. Interacts with SVP26. Golgi localization depends on SVP26. Leads to calcium resistance and vanadate sensitivity. Present with 6730 molecules/cell in log phase SD medium. Belongs to the MNN1/MNT family.