Potassium channel gating observed with site-directed mass tagging.

Potassium channels allow the selective flow of K+ ions across otherwise impermeable membranes. During a process called gating, these channels undergo a conformational change that proceeds from a closed to an open state. The closed state of KcsA, a prokaryotic potassium channel, has been structurally well characterized with equilibrium structural techniques. However, attempts to obtain a structural description of the gating transition of the channel have been hampered because the open state is only transiently occupied and, therefore, not readily accessible to such techniques. Here we describe a non-equilibrium technique that we call site-directed mass tagging and use this technique to probe the conformational change that KcsA undergoes during gating. The results indicate that KcsA is a dynamically modular molecule; the extracellular half of the membrane-spanning region is held rigid during gating, while the intracellular half undergoes a significant conformational change.