Enables mannosyltransferase activity. Involved in GPI anchor biosynthetic process. Predicted to be located in endoplasmic reticulum membrane. Predicted to be integral component of membrane. Predicted to be part of glycosylphosphatidylinositol-mannosyltransferase I complex. Orthologous to human PIGM (phosphatidylinositol glycan anchor biosynthesis class M). [provided by Alliance of Genome Resources, Apr 2022]