Allosteric Properties of KCNQ1 (Kv7.1) Channel Gating Detected by Voltage Clamp Fluorometry

Abstract

KCNQ1 (Kv7.1) is a unique member of the superfamily of voltage gated K+ channels in that it displays a remarkable range of gating behaviors tuned by co-assembly with different β subunits of the KCNE family of proteins. Homomeric KCNQ1 channels activate quickly over a negative range of voltages; KCNQ1/KCNE1 channels activate very slowly over a depolarized range of voltages; and KCNQ1/KCNE3 channels are constitutively open. To better understand the basis for the biophysical diversity of co-assembled channels, we here investigate the basis of KCNQ1 gating in the absence of β subunits using voltage clamp fluorometry (VCF). Based on our previous work, the kinetics and voltage dependence of voltage sensor movements are very similar to those of the channel gate, suggesting a one-to-one relationship. Here, we have tested alterative hypotheses to explain KCNQ1 gating: 1) KCNQ1 voltage sensors undergo a single concerted movement that leads to channel opening, or 2) independent voltage sensor movements lead to channel opening before all voltage sensors have moved. We find that KCNQ1 voltage sensors move independently, but that the channel can conduct before all voltage sensors move. In some mutant KCNQ1 channels, transition to the open state occurs even in the absence of voltage sensor movement. In these mutants, voltage sensors display more depolarized voltage dependence than the channel gate, implying that voltage sensors move after the channel has opened. To interpret these results, we propose an allosteric gating scheme wherein KCNQ1 is able to transition to the open state after 0-4 voltage sensor movements, with each successive voltage sensor movement strengthening the opening transition. This model allows for widely varying gating behavior depending on the relative strength of the opening transition, which physiologically is controlled by co-assembly with different KCNE family members.