Molecular rearrangements in S6 during slow inactivation in Shaker-IR potassium channels

Szanto et al. provide data that support the idea that rearrangement of S6, including its rotation, may mediate the communication between the activation gate and the inactivation gate controlling slow inactivation in K-V channels. Voltage-gated K+ channels have distinct gates that regulate ion flux: the activation gate (A-gate) formed by the bundle crossing of the S6 transmembrane helices and the slow inactivation gate in the selectivity filter. These two gates are bidirectionally coupled. If coupling involves the rearrangement of the S6 transmembrane segment, then we predict state-dependent changes in the accessibility of S6 residues from the water-filled cavity of the channel with gating. To test this, we engineered cysteines, one at a time, at S6 positions A471, L472, and P473 in a T449A Shaker-IR background and determined the accessibility of these cysteines to cysteine-modifying reagents MTSET and MTSEA applied to the cytosolic surface of inside-out patches. We found that neither reagent modified either of the cysteines in the closed or the open state of the channels. On the contrary, A471C and P473C, but not L472C, were modified by MTSEA, but not by MTSET, if applied to inactivated channels with open A-gate (OI state). Our results, combined with earlier studies reporting reduced accessibility of residues I470C and V474C in the inactivated state, strongly suggest that the coupling between the A-gate and the slow inactivation gate is mediated by rearrangements in the S6 segment. The S6 rearrangements are consistent with a rigid rod-like rotation of S6 around its longitudinal axis upon inactivation. S6 rotation and changes in its environment are concomitant events in slow inactivation of Shaker K-V channels.

Automated summary

Szanto et al. provide evidence supporting the communication between the activation gate and the inactivation gate in K-V channels through the rearrangement of S6, including its rotation. By engineering cysteines at specific S6 positions and testing their accessibility to cysteine-modifying reagents, the researchers found that the coupling between the gates is mediated by rearrangements in the S6 segment. These rearrangements involve a rigid rotation of S6 and concomitant changes in its environment during slow inactivation of Shaker K-V channels.