Blocker protection in the pore of a voltage-gated K+ channel and its structural implications

The structure of the bacterial potassium channel KcsA(1) has provided a fra mework for understanding the related voltage-gated potassium channels (Kv c hannels) that are used for signalling in neurons. Opening and closing of th ese Kv channels (gating) occurs at the intracellular entrance to the pore, and this is also the site at which many open channel blockers affect Kv cha nnels(2-4). To learn more about the sites of blocker binding and about the structure of the open Ky channel, we investigated here. the ability of bloc kers to protect against chemical modification of cysteines introduced at si tes in transmembrane segment S6, which contributes to the intracellular ent rance. Within the intracellular half of S6 we found an abrupt cessation of protection for both large and small blockers that is inconsistent with the narrow 'inner pore' seen in the KcsA structure. These and other results are most readily explained by supposing that the structure of Ky channels diff ers from that of the non-voltage-gated bacterial channel by the introductio n of a sharp bend in the inner (S6) helices. This bend would occur at a Pro -X-Pro sequence that is highly conserved in Ky channels, near the site of a ctivation gating.