Functional and molecular characterization of novel Hansenula polymorpha genes, HpPMT5 and HpPMT6, encoding protein O-mannosyltransferases.

•We characterize systematically two novel Hansenula polymorpha genes, HpPMT5 and HpPMT6.•Mutation of these genes in the absence of HpPMT1 generates apparent defects in cell wall integrity.•O-mannosylation of some proteins was defected in the Hppmt1pmt5Δ and Hppmt1pmt6Δ double mutants.•Minor interaction between HpPmt5p and HpPmt2p was detected only in the absence of HpPmt1p.•HpPmt5p and HpPmt6p become evident upon the loss of HpPmt1p for protein O-mannosylation.