Septins coordinate cell wall integrity and lipid metabolism in a sphingolipid-dependent process

Abstract

During normal development and response to environmental stress, fungi must coordinate synthesis of the cell wall and plasma membrane. Septins, small cytoskeletal GTPases, colocalize with membrane sterol-rich regions and facilitate recruitment of cell wall synthases during dynamic wall remodeling. In this study we show that null mutants missing an Aspergillus nidulans core septin present in hexamers and octamers ({Delta}aspAcdc11, {Delta}aspBcdc3, or {Delta}aspCcdc12) are sensitive to multiple cell wall-disturbing agents known to activate the cell wall integrity MAPK pathway and that this sensitivity can be remediated by osmotic support. The null mutant missing the octamer-exclusive core septin ({Delta}aspDcdc10) showed similar osmotic-remedial sensitivity, but only to a single cell wall-disturbing agent and the null mutant missing the noncore septin ({Delta}aspE) showed very mild osmotic-remedial sensitivity to a different single agent. Representative core septin null mutants showed changes in cell wall polysaccharide composition, organization, and chitin synthase localization. Double mutant analysis with{Delta} mpkA suggested core septins interact with the cell wall integrity pathway. Null mutants missing any of the five septins were resistant to ergosterol-disrupting agents. The {Delta}aspAcdc11, {Delta}aspBcdc3, and {Delta}aspCcdc12 mutants showed increased sensitivity to sphingolipid-disrupting agents that was remediated by addition of exogenous phytosphingosine. Representative core septins were mislocalized after treatment with sphingolipid-disrupting agents, but not after treatment with ergosterol-disrupting agents. When challenged with both sphingolipid-disturbing and cell wall-disturbing agents in combination, remediation of the lipid defect restored proper growth to {Delta}aspAcdc11, {Delta}aspBcdc3, and {Delta}aspCcdc12, but remediation of the cell wall defect did not. Our data suggest that the core hexamer and octamer septins are involved in cell wall integrity signaling with the noncore septin playing a minor role; that all five septins are involved in monitoring ergosterol metabolism; that the hexamer septins are required for sphingolipid metabolism; and that septins require sphingolipids to coordinate the cell wall integrity response.