An Experimentally-Validated Model Structure of the Hv1 Proton Channel Voltage Sensor in its Resting State

Abstract

The structure of an Hv1-based chimeric voltage sensor protein (mHv1cc) was recently solved by X-ray crystallography. Although mHv1cc mediates voltage-gated H+ currents when expressed in mammalian cells, the crystallized protein likely represents a closed, possibly resting, conformation. However, the mHv1cc structure is apparently incompatible with our experimental measurements of the resting-state H+ ‘shuttle’ conductance (GSH) in hHv1 R205H. In order to explore possible conformations of the Hv1 voltage sensor (VS) domain in its resting state, we created an Hv1 VS domain homology model (Hv1 D) based on a previously reported resting-state Rosetta model structure of the Shaker K+ channel VS domain (Pathak, et al. 2007) and subjected the Hv1 model to all-atom MD simulations. We subsequently created a model of the Hv1 D R205H point mutant that mediates GSH. In contrast to mHv1cc and a resting-state model of the Ciona Hv1 VS domain (Chamberlin, et al. 2013), the central crevice in our new model is not obviously occluded by hydrophobic side chains. A prominent feature of Hv1 D R205H is that the imidazole ring of the introduced histidine appears to be simultaneously accessible to intra- and extra-cellular aqueous vestiblues, and therefore appears competent for Grotthuss-type proton conduction. Hv1 D represents the first VS domain resting-state model structure that satisfies the rigorous structural constraints imposed by experimental data and thus serves as a template for understanding the structural basis of VS activation in a variety of VS domain-containing proteins.