Abstract
Transient receptor potential mucolipin 1 (TRPML1), a lysosomal channel, maintains the low pH and calcium levels for lysosomal function. Several small molecules modulate TRPML1 activity. ML-SA1, a synthetic agonist, binds to the pore region and phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P2), a natural lipid, stimulates channel activity to a lesser extent than ML-SA1; moreover, PtdIns(4,5)P2, another natural lipid, prevents TRPML1-mediated calcium release. Notably, PtdIns(3,5)P2 and ML-SA1 cooperate further increasing calcium efflux. Here we report the structures of human TRPML1 at pH 5.0 with PtdIns(3,5)P2, PtdIns(4,5)P2, or ML-SA1 and PtdIns(3,5)P2, revealing a unique lipid-binding site. PtdIns(3,5)P2 and PtdIns(4,5)P2 bind to the extended helices of S1, S2, and S3. The phosphate group of PtdIns(3,5)P2 induces Y355 to form a π-cation interaction with R403, moving the S4–S5 linker, thus allosterically activating the channel. Our structures and electrophysiological characterizations reveal an allosteric site and provide molecular insight into how lipids regulate TRP channels.
Highlights
Transient receptor potential mucolipin 1 (TRPML1), a lysosomal channel, maintains the low pH and calcium levels for lysosomal function
Single channel recordings of mouse TRPML1 activity indicated that the open probability of TRPML1 remained quite low when the PtdIns(3,5) P2 was bound, with less than half of the channel population presented in an open conformation[18]
TRPML1 employs its extensions of IS1–IS3 to recognize the PtdInsP2 binders using basic amino acids
Summary
Transient receptor potential mucolipin 1 (TRPML1), a lysosomal channel, maintains the low pH and calcium levels for lysosomal function. Our structures and electrophysiological characterizations reveal an allosteric site and provide molecular insight into how lipids regulate TRP channels. While recent structural determinations provide insight into the regulatory mechanisms of TRPs by small molecules[9,10,11,12], there is still limited structural detail showing how specific lipids regulate activity. We report three structures of human TRPML1 with (1) PtdIns(3,5)P2, (2) PtdIns(4,5)P2, and (3) MLSA1 with PtdIns(3,5)P2 at 3.5–3.7-Å resolution, revealing a lipidbinding site to allosterically activate the channel, distinct from the previously reported site in TRPV19,21 These structures along with electrophysiological characterizations reveal an allosteric site and illuminate how lipids regulate TRP channels
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