The regulatory mechanism of mammalian TRPMLs revealed by cryo-EM

Abstract

Transient receptor potential mucolipin (TRPML) channels are the most recently identified subfamily of TRP channels and have seen a surge of new reports revealing both structural and functional insight. In 2017, several groups published multiple conformations of TRPML channels using cryo-EM. Similar to other TRP channels, the ML subfamily consists of six transmembrane helices (S1-S6), and a pore region including S5, S6, and two pore helices (PH1 and PH2). However, these reports also reveal distinct structural characteristics of the ML subfamily. Asp residues within the luminal pore may function to control calcium/pH regulation. A synthetic agonist, ML-SA1, can bind to the pore region of TRPMLs to force a direct dilation of the lower gate. Finally, biophysical and electrophysiological characterizations reveal another natural agonist binding site in the unique domain of TRPMLs, presumably regulating the conformation of the S4-S5 linker to open the channel. This work elucidates the molecular architecture and provides insights into how multiple ligands regulate TRPMLs.

Keywords: PIP 2; TRPML; ML-SA1; cryo-EM; pH regulation.

Fig. 1

(A) The secondary structure of TRPMLs. (B) The structure of C. jacchus TRPML3 (PDB code: 5W3S). (C) The structure of human TRPML1 with its agonist ML-SA1 (PDB code: 5JW9). (D) Whole-cell current density at −80 mV recorded at pH 4.6 for cells transfected with pEGFP-C1 vector-coding human TRPML1 L/A. Values are mean ± SEM, this panel reproduced from [18].

Fig. 2

(A) detail of the interaction of TRPML1 with ML-SA1. Residues are presented as sticks in green (from the same subunit) and yellow (from the neighboring subunit). (B) Molecular mechanism of V432-mediated TRPML1 regulation. (C) Comparison of pore radius for ML-SA1-bound (blue) and apo TRPML1 structures (red). (D) Structure of resiniferatoxin-bound TRPV1 (PDB code: 5IRX). (E) Structure of UK-59811-bound Ca_VAb (PDB code: 5KLG). This figure reproduced from [18].

Fig. 3

(A) Endogenous lipid-binding sites in TRPML3 structure (PDB code: 5W3S). (B) The putative PI(3,5)P₂-binding site in TRPML3. (C) The structural comparison of the conformations of S4–S5 linker of mouse TRPML1 in the two closed states (5WPQ and 5WPT).