Mutations of the S4-S5 linker alter activation properties of HERG potassium channels expressed in Xenopus oocytes

1. The structural basis for the activation gate of voltage-dependent K+ cha nnels is not known, but indirect evidence has implicated the S4-S5 Linker, the cytoplasmic region between the fourth and fifth transmembrane domains o f the channel subunit. We have studied the effects of mutations in the S4-S 5 Linker of HERG (human ether-a-go-go-related gene), a human delayed rectif ier K+ channel, in Xenopus oocytes. 2. Mutation of acidic residues (D540, E544) in the S4-S5 linker of HERG cha nnels to neutral (Ala) or basic (Lys) residues accelerated the rate of chan nel deactivation. Most mutations greatly accelerated the rate of activation . However, E544K HERG channels activated more slowly than wild-type HERG ch annels. 3. Mutation of residues in the S4-S5 linker had Little or no effect on fast inactivation, consistent with independence of HERG channel activation and inactivation 4. In response to large hyperpolarizations, D540K HERG channels can reopen into a state that is distinct from the normal depolarization-induced open s tate. It is proposed that substitution of a negatively charged Asp with the positively charged Lys disrupts a subunit interaction that normally stabil izes the channel in a closed state at negative transmembrane potentials. 5. The results indicate that the S4-S5 Linker is a crucial component of the activation gate of HERG channels.