Phospholipid scrambling by a TMEM16 homolog of Arabidopsis thaliana

Membrane asymmetry is important for cellular physiology and established by energy-dependent unidirectional lipid translocases, which have diverse physiological functions in plants. By contrast, the role of phospholipid scrambling (PLS), the passive bidirectional lipid transfer leading to the break-down of membrane asymmetry, is currently still unexplored. The Arabidopsis thaliana genome contains a single gene (At1g73020) with homology to the eukaryotic TMEM16 family of Ca2+-activated phospholipid scramblases. Here, we investigated the protein function of this Arabidopsis homolog. Fluorescent AtTMEM16 fusions localized to the ER both in transiently expressing Arabidopsis protoplasts and HEK293 cells. A putative scrambling domain (SCRD) was identified on the basis of sequence conservation and conferred PLS to transfected HEK293 cells, when grafted into the backbone of the non-scrambling plasma membrane-localized TMEM16A chloride channel. Finally, AtTMEM16 `gain-of-function' variants gave rise to cellular phenotypes typical of aberrant scramblase activity, which were reversed by the additional introduction of a `loss-of-function' mutation into the SCRD. In conclusion, our data suggest AtTMEM16 works as an ER-resident lipid scramblase in Arabidopsis.