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. Promotes iron uptake and usage along the endocytosis pathway under iron-limiting conditions. Enzyme activity is regulated by iron. Protein modification; protein glycosylation. Decreases the ability to extend O-linked, and possibly also N-linked, mannans. Leads to hypersensitivity to cell wall-damaging agents, and to a reduction of cell wall mannosylphosphate. Leads also to resistance to killing by the iron-chelating protein lactoferrin. Belongs to the MNN1/MNT family.