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Elife. 2019 Feb 21;8. pii: e44364. doi: 10.7554/eLife.44364.

Stepwise activation mechanism of the scramblase nhTMEM16 revealed by cryo-EM.

Author information

1
Department of Biochemistry, University of Zurich, Zurich, Switzerland.
2
Department of Structural Biology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Groningen, The Netherlands.

Abstract

Scramblases catalyze the movement of lipids between both leaflets of a bilayer. Whereas the X-ray structure of the protein nhTMEM16 has previously revealed the architecture of a Ca2+-dependent lipid scramblase, its regulation mechanism has remained elusive. Here, we have used cryo-electron microscopy and functional assays to address this question. Ca2+-bound and Ca2+-free conformations of nhTMEM16 in detergent and lipid nanodiscs illustrate the interactions with its environment and they reveal the conformational changes underlying its activation. In this process, Ca2+ binding induces a stepwise transition of the catalytic subunit cavity, converting a closed cavity that is shielded from the membrane in the absence of ligand, into a polar furrow that becomes accessible to lipid headgroups in the Ca2+-bound state. Additionally, our structures demonstrate how nhTMEM16 distorts the membrane at both entrances of the subunit cavity, thereby decreasing the energy barrier for lipid movement.

KEYWORDS:

S. cerevisiae; TMEM16; activation mechanism; biochemistry; chemical biology; cryo-EM; lipid scrambling; membrane interaction; molecular biophysics; structural biology

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