Essential role of non-vesicular lipid transport in microtubule-controlled cell polarisation

<jats:title>Abstract</jats:title><jats:p>Cell polarisation is a fundamental biological process. Fission yeast is a key model system to study the molecular basis of microtubule-controlled cell polarisation. In this process, cells define prospective growth sites by generating distinct plasma membrane domains enriched in <jats:italic>de novo</jats:italic> synthesised sterols. Microtubules restrict the number and location of these domains by depositing factors at the cell poles. The mechanisms underlying such sterol-rich membrane domain formation and polarisation are largely unknown. We found that the oxysterol-binding proteins kes1p, osh2p and kes3p define three independent sterol delivery pathways to the plasma membrane. These mediate different phases of cell polarisation in a phosphoinositide-dependent fashion and differ in their requirement for vesicular trafficking steps. The redundant, kes1p- and osh2p-dependent pathways are vital and prime cell polarisation by mediating the formation of randomly distributed sterol-rich plasma membrane domains. Subsequent microtubule-controlled polarisation of these domains preferentially employs kes1p that directly delivers sterols to the plasma membrane independent of cdc42p. In cells lacking kes1p, polarisation becomes cdc42p-dependent, utilising mainly the kes3p-dependent pathway. Our study uncovers an essential biological function for non-vesicular lipid transport and establishes a molecular basis for different sterol-delivery pathways acting in cdc42p-independent and cdc42p-dependent cell polarisation.</jats:p>