A different mechanism of C-type inactivation in the Kv-like KcsA mutant E71V
A large class of K+ channels display a time-dependent phenomenon called C-type inactivation whereby prolonged activation by an external stimulus leads to a non-conductive conformation of the selectivity filter. C-type inactivation is of great physiological importance particularly in voltage-activated K+ channels (Kv), affecting the firing patterns of neurons and shaping cardiac action potentials. While understanding the molecular basis of inactivation has a direct impact on human health, its structural basis remains unresolved. Knowledge about C-type inactivation has been largely deduced from the pH-activated bacterial K+ channel KcsA, whose selectivity filter under inactivating conditions adopts a constricted conformation at the level of the central glycine (TTVGYGD) that is stabilized by tightly bound water molecules. However, C-type inactivation is highly sensitive to the molecular environment surrounding the selectivity filter in the pore domain, which is different in Kv channels than in the model KcsA. In particular, a glutamic acid residue at position 71 along the pore helix in KcsA is consistently substituted by a nonpolar valine in most Kv channels, suggesting that this side chain is an important molecular determinant of function. Here, a combination of X-ray crystallography, solid-state NMR and molecular dynamics simulations of the E71V mutant of KcsA is undertaken to explore the features associated with this Kv-like construct. In both X-ray and ssNMR data, it is observed that the filter of the Kv-like KcsA mutant does not adopt the familiar constricted conformation under inactivating conditions. Rather, the filter appears to adopt a conformation that is slightly narrowed and rigidified over its entire length. No structural inactivation water molecules are present. On the other hand, molecular dynamics simulations indicate that the familiar constricted conformation can nonetheless be stably established in the mutant channel. Together, these findings suggest that the Kv-like E71V mutation in the KcsA channel may be associated with different modes of C-type inactivation, showing that distinct selectivity filter environments entail distinct C-type inactivation mechanisms.