Ca2+-dependent phospholipid scrambling by a reconstituted TMEM16 ion channel

Abstract

Phospholipid (PL) scramblases disrupt the lipid asymmetry of the plasma membrane, externalizing phosphatidylserine to trigger blood coagulation and mark apoptotic cells. Recently, members of the TMEM16 family of Ca2+-gated channels have been shown to be involved in Ca2+-dependent scrambling. It is however controversial whether they are scramblases or channels regulating scrambling. Here we show that purified afTMEM16, from Aspergillus fumigatus, is a dual-function protein: it is a Ca2+-gated channel, with characteristics of other TMEM16 homologues, and a Ca2+-dependent scramblase, with the expected properties of mammalian PL scramblases. Remarkably, we find that a single Ca2+ site regulates separate transmembrane pathways for ions and lipids. Two other purified TMEM16-channel homologues do not mediate scrambling, suggesting that the family diverged into channels and channel/scramblases. We propose that the spatial separation of the ion and lipid pathways underlies the evolutionary divergence of the TMEM16 family, and that other homologues, such as TMEM16F, might also be dual-function channel/scramblases. TMEM16-channel family members have been shown to be involved in Ca2+-dependent lipid scrambling, but whether they have intrinsic scramblase activity remains controversial. Malvezzi et al. identify a TMEM16 family member in Aspergillus in which a single Ca2+-binding site regulates intrinsic channel and scramblase activities.