Intracellular Helix-Loop-Helix Domain Modulates Inactivation Kinetics of Mammalian TRPV5 and TRPV6 Channels

TRPV5 and TRPV6 are calcium-selective ion channels expressed at the apical membrane of epithelial cells. Important for systemic calcium (Ca2+) homeostasis, these channels are considered gatekeepers of this cation transcellular transport. Intracellular Ca2+ exerts a negative control over the activity of these channels by promoting inactivation. TRPV5 and TRPV6 inactivation has been divided into fast and slow phases based on their kinetics. While slow inactivation is common to both channels, fast inactivation is characteristic of TRPV6. It has been proposed that the fast phase depends on Ca2+ binding and that the slow phase depends on the binding of the Ca2+/Calmodulin complex to the internal gate of the channels. Here, by means of structural analyses, site-directed mutagenesis, electrophysiology, and molecular dynamic simulations, we identified a specific set of amino acids and interactions that determine the inactivation kinetics of mammalian TRPV5 and TRPV6 channels. We propose that the association between the intracellular helix-loop-helix (HLH) domain and the TRP domain helix (TDh) favors the faster inactivation kinetics observed in mammalian TRPV6 channels.

Automated summary

TRPV5 and TRPV6 are calcium-selective ion channels that play a crucial role in maintaining systemic calcium homeostasis. These channels are regulated by intracellular calcium, which induces inactivation. The inactivation of TRPV5 and TRPV6 can be divided into fast and slow phases, with the fast inactivation being unique to TRPV6. By utilizing various techniques, including structural analyses and mutagenesis, we have identified specific amino acids and interactions that govern the inactivation kinetics of mammalian TRPV5 and TRPV6 channels. Additionally, we propose that the association between the intracellular HLH domain and the TRP domain helix contributes to the faster inactivation kinetics observed in mammalian TRPV6 channels.