Transfers mannose from GDP-mannose to dolichol monophosphate to form dolichol phosphate mannose (Dol-P-Man) which is the mannosyl donor in pathways leading to N-glycosylation, glycosyl phosphatidylinositol membrane anchoring, and O-mannosylation of proteins; catalytic subunit of the dolichol-phosphate mannose (DPM) synthase complex. a dolichyl phosphate + GDP-alpha-D-mannose = a dolichyl beta-D-mannosyl phosphate + GDP Protein modification; protein glycosylation. Component of the dolichol-phosphate mannose (DPM) synthase complex composed of DPM1, DPM2 and DPM3; within the complex, directly interacts with DPM3. This interaction may stabilize DPM1. Belongs to the glycosyltransferase 2 family.