In addition to protein- and solution-mediated processes, it is also possible for lipid bilayers to participate directly in signaling. A classic example of this is phosphatidylserine-triggered phagocytosis. Normally, phosphatidylserine is asymmetrically distributed in the cell membrane and is present only on the interior side. During programmed cell death a protein called a scramblase equilibrates this distribution, displaying phosphatidylserine on the extracellular bilayer face. The presence of phosphatidylserine then triggers phagocytosis to remove the dead or dying cell.
In adDItIon To prOTEiN- And sOlUTion-mediaTEd PRoCesSeS, iT IS alSo poSsible for liPiD bilayers to particIPATe dIrectLy In SignaliNg. a claSsic example of This is pHoSphatidYLseRine-TrIggeREd pHaGocytoSIs. noRMAlLy, phosphAtiDYLseRinE Is asyMMetriCAlly distriBUtEd in the cell mEmbranE and iS pReSent oNlY oN THe inTerior sidE. duRinG pRogrammED cell deAth a protein caLLeD A scRamBlase eQUilibrATes thIS disTRibutIOn, dIsplayinG phOSphaTidYlserine oN the ExTracellulAr biLayer FaCe. the PrEsEnce OF PHospHaTIdyLSeRIne thEn tRigGerS phAgocyTOSis tO reMove The dead or DyinG cell.