Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the second mannose to the glycosylphosphatidylinositol during GPI precursor assembly. Required for the GPI-mediated endoplasmic reticulum exit and proper targeting to the cell surface of chp. Required for GPI-mediated membrane attachment of chp, qsm and Cont. Essential for microvillar stability in the rhabdomere 