Protein O-mannosyltransferase involved in O-glycosylation which is essential for cell wall rigidity. Forms a heterodimeric complex with PMT2 and more rarely with PMT3 to transfer mannose from Dol-P-mannose to Ser or Thr residues on proteins. The PMT1-PMT2 complex participates in oxidative protein folding, ER-associated protein degradation (ERAD), as well as ER export. Required for incorporation of proteins in the cell wall. 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. PMT1 and PMT2 form a functional heterodimer. The complex interacts with endoplasmic reticulum proteins EMP24, ERV25, ERP1, ERP2, CDC48, HRD1, USA1, YOS9, ERO1, PDI1, UBR1, Cue4, DFM1 and TED1. Forms also a minor complex with PMT3. The large luminal loop 5 is essential for mannosyltransferase activity but not for PMT1-PMT2 complex formation. Affects glycosylation of chitinase and increases sensitivity to calcofluor white and caffeine. Present with 41500 molecules/cell in log phase SD medium. Belongs to the glycosyltransferase 39 family.