Dissection of the structure-function relationship of Na channels.

Voltage-gated sodium channels (Na) undergo conformational shifts in response to membrane potential changes, a mechanism known as the electromechanical coupling. To delineate the structure-function relationship of human Na channels, we have performed systematic structural analysis using human Na1.7 as a prototype. Guided by the structural differences between wild-type (WT) Na1.7 and an eleven mutation-containing variant, designated Na1.7-M11, we generated three additional intermediate mutants and solved their structures at overall resolutions of 2.9-3.4 Å. The mutant with nine-point mutations in the pore domain (PD), named Na1.7-M9, has a reduced cavity volume and a sealed gate, with all voltage-sensing domains (VSDs) remaining up. Structural comparison of WT and Na1.7-M9 pinpoints two residues that may be critical to the tightening of the PD. However, the variant containing these two mutations, Na1.7-M2, or even in combination with two additional mutations in the VSDs, named Na1.7-M4, failed to tighten the PD. Our structural analysis reveals a tendency of PD contraction correlated with the right shift of the static inactivation I-V curves. We predict that the channel in the resting state should have a "tight" PD with down VSDs.