Modular Thermal Sensors in Temperature-Gated TRP Channels

Abstract

A group of ion channels in the TRP family, the so-called thermal TRP channels, exhibit unprecedentedly strong temperature dependence, some of which reach Q10 > 100 as compared to 2-3 for typical enzymatic reactions. The strong thermal sensitivity of these channels arises because their gating involves large enthalpy changes between closed and open. To quantify the energetics of gating, we applied optically generated submillisecond temperature jumps to directly measure the temperature-dependent activation kinetics. The data show that the opening of TRPV1, for example, is accompanied with an enthalpy change of ∼100 kcal/mol, five times the energetic changes in voltage- or ligand-dependent gating. To gain insights on the energetic source, we analyzed single-channel and macroscopic kinetics of temperature-dependent gating, both showing that temperature has localized effects on specific gating components. Furthermore, the perturbation of membrane compositions, which altered the physical properties of lipids such as temperature-dependent phase transitions, did not abolish temperature activation of the channel. Thus the thermal sensitivity of the channel appears to be intrinsic to the channel itself, most likely arising from a specific protein domain rather than integration of global thermal effects. Using systematic chimeric analysis, we uncovered a proximal N-terminal region to be crucial for temperature sensing in heat-activated TRPVs. Changing this region both successfully transferred thermal sensitivity to temperature-insensitive isoforms and profoundly altered thermal sensing in temperature-sensitive wild-type channels. Swapping other domains including the whole transmembrane core, the C terminus, and the rest of the N terminus had little effect on the large enthalpy of gating. These results support that thermal TRP channels contain modular thermal sensors for their activation by temperature.