A Non-canonical VSD-Pore Coupling is Responsible for the AO State of KCNQ1 Channels

Abstract

In voltage dependent activation of Kv channels, membrane depolarization activates the voltage sensor(VSD), which then opens the pore via the VSD-pore coupling. The interaction between the S4-S5 linker and the cytosolic end of S6 has been proposed to mediate the VSD-pore coupling. KCNQ1 (Kv7.1) in association with the auxiliary subunit KCNE1 forms the IKs channel in the heart that is critical for controlling heart rhythm. It has been shown that KCNQ1 opens to two open states when the voltage sensor is activated to the intermediate state (termed IO) and the fully activated state (AO), respectively. The association of KCNE1 suppresses the IO state and enhances the AO state. Therefore, the IKschannel only opens to the AO state, which is more important to the cardiac physiology. We have previously shown that the F351A mutation selectively suppressed the IO state by disrupting the VSD-pore coupling. Since F351 is a conserved residue important for the above mentioned VSD-pore coupling mechanism, this result suggests that the canonical coupling mechanism is important for the IO state, In this study we find that three mutations of KCNQ1 that are associated with Long QT syndrome, S338F, F339S and L342A disrupt the VSD-pore coupling to selectively eliminate the AO state. These residues are located in the middle of S6 and cannot interact with the S4-S5 linker. These results indicate that different VSD-pore coupling mechanisms are responsible for the IO and AO states, and mutations in different structural motifs can selectively disrupt either of the mechanisms. Furthermore, the middle of S6 is involved in a non-canonical mechanism that specifically couples VSD activation to the AO state.