Localization of an Alternative Ion Permeation Pathway in TRPM3

Abstract

Transient receptor potential (TRP) channels are cationic channels that are involved in a broad array of functions, including homeostasis, motility and sensory functions such as taste transduction, nociception, and temperature sensing. In general, the activation of these channels leads to the opening of their central channel pore resulting in a cationic influx and a subsequent depolarization of the cell. TRP melastatin 3 (TRPM3) is a member of the melastatin family of TRP channels and was shown to be expressed in sensory neurons where it acts as noxious heat sensor and nociceptor. Activation of TRPM3 results typically in an outward-rectifying current-voltage (I-V) curve due to the opening of the central pore of the channel. Recently, it was shown that certain agonists can induce the opening of an alternative ion permeation pathway next to the central pore, resulting in outward- and inward-rectifying currents. Here we use mutagenesis studies to localize the alternative pore of TRPM3. Our data suggest the location of the alternative pore in the voltage-sensing domain (VSD) of the channel and more specifically in proximity to transmembrane segment 4 (S4). We show three residues (W982, D988 and G991) that seem to be involved in the shaping of the alternative ion permeation pathway. The results obtained in this study are in line with the location of other non-canonical pores described in mutated voltage-gated K+-, Na+- and Ca2+-channels and propose the potential occurrence of non-canonical pores in other ion channels that are lacking the typical gating charges found in voltage-gated channels.