SK-channels (SKs) are small-conductance calcium-activated potassium channels that are involved in the hyperpolarization of neurons and other excitable cells. Calmodulin, a calcium-sensing protein, is complexed with these potassium channels. Upon calcium-binding, calmodulin undergoes a conformational change that ultimately opens the channel, allowing potassium through. The human genome encodes four different SK-channels (SK1-4). Each paralog is found in varying concentrations throughout the body, with varying effects on the brain and heart. Despite this, the activation mechanism has remained elusive until recently when it was shown how the human SK4 and calmodulin conformations change depending on the binding of calcium to calmodulin. In the inactive state, when calmodulin has not bound calcium, calmodulin is only partially bound to the channel. In the semi-active state, calmodulin is fully bound to the channel but the channel is closed. In the fully-active state, the channel is now open to transport potassium across the membrane. By mapping the interface residues found in each of the three conformations, I identified the residues important for the interactions between subunits. This was combined with an examination of the molecular evolution of this complex across vertebrates to elucidate the conservation of the activation mechanism for the four SK clades and their interactions with calmodulin in the different conformations. The channel-calmodulin interface was found to have more radical changes than within the channel, suggesting that the mechanism of activation by calmodulin may have diverged between the different SK channel paralogs, while the mechanism of channel activation is conserved. The divergence in activation by calmodulin can be explored as a way to therapeutically regulate the activation of the SK channels in a paralog specific manner.
