Do Ion Channels Spin?

The interior of a living cell is maintained at a different electrical potential than the exterior, by differences in the concentration of various ion species [1]. Changes in this voltage, during action potentials for example, occur via ion flows through specialized pores in the cell membrane. The more rapid the ionic flows through the pores, and the more quickly they can be gated, the more efficient the overall system can be. Recent advances in imaging pore proteins at the nanoscale have shown that pores are not just simple holes through the membrane, but are sculpted structures, particularly on the intracellular side of the membrane. For example, the elegant images of Sokolova et al [2] of the potassium Shaker channel show a “hanging basket” structure decorating the interior side of the membrane (Fig. 1). Recent images of sodium channel proteins by Sato and colleagues [3] and Payandeh et al [4] again display not just a simple hole, but an intricate series of tubes, suggesting a nanofluidic device (Fig. 2). In each of these examples there are lateral tubes, lying parallel to the membrane surface. What are the functions of these shapes? As an evolved form, they need not have any single “function”. But one can still speculate as to how a particular shape might facilitate the task of gating a stream of ions from one side of a membrane to the other. We will suggest that as part of this task, some membrane pore proteins may be in rapid rotary motion. By simple angular momentum arguments, if ions and water molecules follow any sort of spiral path through the membrane pore, rotary motion of the pore protein itself is not only possible, but likely. Further, ions driven by an electrical field along a helical path can generate considerable torque acting on the structure as a whole. The motif of a spinning biochemical rotor is by now familiar. Besides bacterial flagella, we have the example of ATP synthase, a catalyst which operates a rotating motor on a molecular scale [5]. So if it can be demonstrated that ionic pores rotate during conduction, we simply have another example of a known architecture, albeit one that perhaps at least temporarily takes over the title of “smallest biological motor”.