Dynamics of activation in the voltage-sensing domain of Ci-VSP

TheCiona intestinalisvoltage-sensing phosphatase (Ci-VSP) is a membrane protein containing a voltagesensing domain (VSD) that is homologous to VSDs from voltage-gated ion channels responsible for cellular excitability. Two crystal structures of Ci-VSD in putative resting and active conformations suggest a helical-screw voltage sensing mechanism in which the S4 helix translocates and rotates to enable exchange of salt-bridge partners. By combining extensive molecular dynamics simulations with a computational framework based on dynamical operators, we elucidate the microscopic mechanism of the resting-active transition at physiological membrane potential. Sparse regression reveals a small set of coordinates that distinguish intermediates hidden from electrophysiological measurements. The intermediates arise from a noncanonical helical-screw mechanism in which translocation, rotation, and side-chain movement of the S4 helix are only loosely coupled. These results provide new insights into existing experimental and computational findings on voltage sensing and suggest ways of further probing its mechanism.