Probing ion configurations in the KcsA selectivity filter using 2D IR spectroscopy

Potassium (K+) ion channels are transmembrane proteins that regulate the passage of K+

ions through cell membranes. The selectivity filter is the narrowest part of the pathway of

ions through the channel. It plays a determining role in the remarkably high ion selectivity

and transport rates. Despite decades of work the precise mechanistic details of the transport

through ion channels are still elusive. According to the so-called soft-knock mechanism

water molecules alternate between K+ ions in the selectivity filter and co-transport with the

ions. In contrast, the hard-knock mechanism assumes that water is absent from the

selectivity filter during ion conduction. Two-dimensional infrared spectroscopy (2D IR) is an

ultrafast technique that measures molecular vibrations. Carbonyl stretching vibrations of a

protein backbone are sensitive probes of the local chemical environment and can be used to

discriminate between water and K+ ions in the selectivity filter.

I will present our recent line shape simulations and experiments, performed by our

collaborators, on a prokaryotic K+ channel KcsA. Our results are clearly consistent with all

the previous 2D IR experiments and illustrate the prevalence of the soft-knock

ion configurations in the closed conductive state of the KcsA channel.