Crystallizing membrane proteins is no easy task. Rod MacKinnon and his co-workers, however, are true master craftsmen in such bold ventures. They were the first to report the crystal structure of a bacterial potassium channel, a pioneering accomplishment for which MacKinnon was awarded the Nobel prize. Although this and subsequent crystal structures of prokaryotic channels provided first insights into how potassium channels work, many questions remained. In a recent pair of Science papers, MacKinnon and his colleagues present the first crystal structure of a mammalian voltage-dependent potassium channel, Kv1.2, and substantially refine the current model of how these channels open and close.
In nerve cells, voltage-dependent potassium channels are involved in the generation of electrical impulses. They are membrane proteins composed of fourα-subunits. Each single α-subunit contains six membrane-spanningα-helices (S1-6), two of which form a central pore (S5, S6) through which potassium ions selectively pass. In addition, regulatory β-subunits are attached to each α-subunit from the cytoplasmic side. In order to react to changes in the membrane potential, voltage-dependent potassium channels are equipped with positively charged sensors lying within the membrane. Most current models propose that these voltage sensors move in response to an altered electric field across the membrane and thus perform mechanical work that influences channel conductivity.
Precisely how the sensors move has been investigated intensively, but the data from different experimental approaches have not yielded a uniform picture. Even the available crystal structures of potassium channels could not elucidate the matter, because the region of the voltage sensor was barely resolved. Previous attempts by the MacKinnon lab to stabilize this region using antibody fragments did not reveal satisfying results because the voltage sensors, although clearer, were found in artificial positions, evidently contradicting solid electrophysiological data.
To stabilize the sensors without the use of antibodies, MacKinnon and his colleagues grew the crystal of the Kv1.2 channel in complex with theβ-subunits in a mixture of lipids and detergents. The crystal allowed the team to determine the channel structure in its open position at a resolution of 2.9 angstroms, which is sufficient to deduce an atomic model of the protein. To the gratification of many, the new structure confirmed most expectations. In particular, the voltage sensors were now largely visible in their native conformation. As previously suggested by the authors, the sensor unit appears to be a highly mobile `paddle' formed by helices S3 and S4 that protrudes partly into the extracellular space. This may explain why Kv1.2 is susceptible to certain spider venoms that are known to poison the voltage sensor from the outside. Voltage-dependent movements of the sensor are coupled to pore opening and closing by a transverse linker-helix between S4 and S5. Its position determines the pore diameter by pushing or pulling the inner pore helix S6. Interestingly, in the open conformation, the sensor's charge-carrying S4 helix is partly exposed to lipids that presumably fill the space between pore- and sensor-helices. However, some of the S4 charges are also shielded from the low dielectric lipid environment by helices S1 and S2,which may be an essential requirement for sensor functioning.
MacKinnon's new potassium channel structure has important implications for our understanding of voltage-sensing and electromechanical coupling. But a static snapshot of the channel's open conformation cannot demonstrate to what extent the voltage sensor moves within the membrane. Therefore, we impatiently await the MacKinnon lab's next snapshot showing the channel in its closed conformation.