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

BioRxiv : the Preprint Server for Biology
Valeria KalienkovaCristina Paulino

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. Ligand-bound and ligand- free conformations of nhTMEM16 in detergents 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 ligand-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.

Related Concepts

Environment
Ligands
Lipids
Electron Microscopy
Ca2+-binding protein-
Calcium ion
Membrane
Cryoelectron Microscopy
Structure
Ligand Binding Domain

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