Transfers mannose from Dol-P-mannose to Ser or Thr residues on proteins (Probable). Mannosylates an artificial substrate, probably on a Thr residue, upon expression in M.smegmatis. Glycosylation probably requires the Sec-translocation system. a dolichyl beta-D-mannosyl phosphate + L-seryl-[protein] = 3-O-(alpha-D-mannosyl)-L-seryl-[protein] + a dolichyl phosphate + H(+) a dolichyl beta-D-mannosyl phosphate + L-threonyl-[protein] = 3-O-(alpha-D-mannosyl)-L-threonyl-[protein] + a dolichyl phosphate + H(+) Protein modification; protein glycosylation. Belongs to the glycosyltransferase 39 family. Truncated N-terminus.