Aspergillus nidulans protein O-mannosyltransferases play roles in cell wall integrity and developmental patterning

Protein O-mannosyltransferases (Pmts) initiate O-mannosyl glycan biosynthesis from Ser and Thr residues of target proteins. Fungal Pmts are divided into three subfamilies, Pmt1, -2, and -4. Aspergillus nidulans possesses a single representative of each Pmt subfamily, pmtA (subfamily 2), pmtB (subfamily 1), and pmtC (subfamily 4). In this work, we show that single Deltapmt mutants are viable and have unique phenotypes and that the DeltapmtA DeltapmtB double mutant is the only viable double mutant. This makes A. nidulans the first fungus in which all members of individual Pmt subfamilies can be deleted without loss of viability. At elevated temperatures, all A. nidulans Deltapmt mutants show cell wall-associated defects and increased sensitivity to cell wall-perturbing agents. The Deltapmt mutants also show defects in developmental patterning. Germ tube emergence is early in DeltapmtA and more frequent in DeltapmtC mutants than in the wild type. In DeltapmtB mutants, intrahyphal hyphae develop. All Deltapmt mutants show distinct conidiophore defects. The DeltapmtA strain has swollen vesicles and conidiogenous cells, the DeltapmtB strain has swollen conidiophore stalks, and the DeltapmtC strain has dramatically elongated conidiophore stalks. We also show that AN5660, an ortholog of Saccharomyces cerevisiae Wsc1p, is modified by PmtA and PmtC. The Deltapmt phenotypes at elevated temperatures, increased sensitivity to cell wall-perturbing agents and restoration to wild-type growth with osmoticum suggest that A. nidulans Pmts modify proteins in the cell wall integrity pathway. The altered developmental patterns in Deltapmt mutants suggest that A. nidulans Pmts modify proteins that serve as spatial cues.