Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the first alpha-1,4-mannose to GlcN-acyl-PI during GPI precursor assembly. Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. A number of isoforms are produced. According to EST sequences. Expressed in influorescence meristem. Present at low level in roots, hypocotyls, leaves and stems. Plants display cell walls with decreased crystalline cellulose, increased pectins and irregular and ectopic deposition of pectins, xyloglucans and callose. Belongs to the PIGM family.