Isolation ofMutant Saccharomyces cerevisiae Strains ThatSurvive without Sphingolipids

strain that requires theaddition ofthelong-chain-base component ofsphingolipids totheculture mediumforgrowth, weisolated twostrains carrying secondary, suppressor mutations thatpermit survival intheabsence ofexogenous long-chain base. Remarkably, thesuppressor strains madelittle ifanysphingolipid. A study ofhowthesuppressor geneproducts compensate forthelack ofsphingolipids mayreveal thefunction(s) ofthese membrane lipids inyeast cells. Sphingolipids constitute adiverse groupofcomplex membrane lipids. Allcontain along-chain aminoalcohol, usually 18carbons, connected byanamidelinkage toafatty acid to formceramide. Thehydroxyl atthe1position ofceramide canbelinked tovarious polar headgroups, including phosphorylcholine (sphingomyelin), ortoglucose units andother moieties (4). Sphingolipids arefound mainly ineucaryotes, wheretheyareubiquitous, withhundreds having been identified andstructurally characterized inmammalsalone (4). Morethanacentury ago,Thudichum (17) discovered the long-chain-base sphingosine inthehumanbrain andnamedit after thesphinx because oftheriddles itposed. Thename remains appropriate. Although sphingolipids areknownto play astructural role inmembranes, their precise biological function(s) isunknown. Ourlack ofknowledge isdueinpart todifficulties inassigning functions whichstemfromthe diversity ofsphingolipids present inmostcell types. Roles havebeenproposed insuchcomplex phenomena ascellular differentiation, cell-to-cell recognition, andmodulation of growth (4,10,12), including involvement inmembrane signal transduction pathways (5). We havebeguntoexamine the function ofsphingolipids intheyeast Saccharomyces cerevisiae, whichiswellsuited forthese studies because sphingolipid diversity isminimized inthat S.cerevisiae contains onlythree abundant sphingolipids, withthefollowing compositions: inositol P-ceramide (IPC), mannose-inositol Pceramide (MIPC)and mannose-(inositol P)2-ceramide