Temperature and pressure dependence of Shaker K+ channel N- and C-type inactivation

Shaker B potassium channels undergo rapid N-type and slow C-type inactivation. While N-type inactivation is supposed to be mediated by occlusion of the pore by the N-terminal protein structure, the molecular mechanisms leading to C-type inactivation are less well understood. Considering N-type inactivation as a model for a protein conformational transition, we investigated inactivation of heterologously expressed Shaker B potassium channels and mutants thereof, showing various degrees of C-type inactivation, under high hydrostatic (oil) pressure. In addition to the derived apparent activation and reaction volumes (delta V), experiments at various temperatures yielded estimates for enthalpic (delta H) and entropic (T delta S) contributions. N-type inactivation was accelerated by increasing temperature and slowed by high hydrostatic pressure yielding at equilibrium delta H = 76 kJ/mole, T delta S = 82 kJ/mole, and delta V = 0.18 nm3 indicating that the transition to the N-type inactivated state is accompanied by an increase in volume and a decrease in order. N-terminally deleted Sh delta 6-46 constructs with additional mutations at either position 449 or 463 were used to investigate C-type inactivation. In particular at high temperatures, inactivation occurred in two phases indicating more than one process. At equilibrium the following values were estimated for the major inactivation component of mutant Sh delta 6-46 T449A: delta H = -64 kJ/mole, T delta S = -60 kJ/mole, and delta V = -0.25 nm3, indicating that the C-type inactivated state occupies a smaller volume and is more ordered than the noninactivated state. Thus, hydrostatic pressure affects N- and C-type inactivation in opposite ways.