TRP-ML1 Is a Lysosomal Monovalent Cation Channel That Undergoes Proteolytic Cleavage

Abstract

Mutations in the gene Mucolipidosis type IV (MLIV)2 is a lipid storage disorder characterized by an abnormal accumulation of membranous lipids in patients' cells (reviewed in Refs. 1 and 2). Clinically, the disease manifests as corneal clouding, degeneration of the retina, and severe psychomotor retardation (1-6). MLIV is associated with mutations in MCOLN1 (TRP-ML1), a member of the TRP (transient receptor potential) family of ion channels (7-9). The TRP family includes several members that are implicated in human diseases, such as TRPP2 (10), TRPM1 (11), and TRPV6 (12). A critical question in MLIV pathogenesis is why do mutations in TRP-ML1 lead to the cellular phenotype of MLIV?Previous work on the ion selectivity and permeation of TRP-ML1 produced conflicting results. Thus, transient expression in Xenopus oocytes and in fibroblasts suggests that TRP-ML1 is targeted to the lysosomes and functions as a Ca 2ϩ -permeable channel that may regulate lysosomal Ca 2ϩ release and consequently agonist-evoked Ca 2ϩ signals (13,14). On the other hand, TRP-ML1 synthesized in cell-free system and reconstituted into planar lipid bilayers behaves as a monovalent cations permeable, outwardly rectifying channel (15). The outward rectification indicates that when present in lysosomes, TRP-ML1 primarily moves ions into the lysosomal lumen. The outward rectification makes it unlikely that in vivo TRP-ML1 would function as a lysosomal Ca 2ϩ release channel, which suggested an alternative role of TRP-ML1 in lysosomal and cellular functions.In the present report we analyzed the expression pattern and channel properties of TRP-ML1 and several disease-associated mutants. We report that TRP-ML1 is an outwardly rectifying monovalent cationpermeable channel that is primarily expressed in the lysosomes. In the lysosomes, TRP-ML1 is inactivated by proteolytic cleavage. These findings suggest a novel mechanism of regulating TRP-ML1 function.

Identification of the Penta-EF-hand Protein ALG-2 as a Ca2+-dependent Interactor of Mucolipin-1

“…As seen in Fig. 6B, both proteins were similarly expressed and were able to oligomerize (upper bands) and undergo proteolytic cleavage (lower band) (17,18,64).…”

“…Post-translational modifications play an important role in the regulation of MCOLN1 function. Palmitoylation and phosphorylation at the carboxyl-terminal tail modulate trafficking and channel activity, respectively (16,17), although cleavage at the first luminal loop inactivates the protein (18). The selectivity of the MCOLN1 channel remains controversial, as different studies have suggested that the channel is permeable to Ca 2ϩ (19), K ϩ , Ca 2ϩ , and Na ϩ (20), H ϩ (21), and Fe 2ϩ (22).…”

A small molecule restores function to TRPML1 mutant isoforms responsible for mucolipidosis type IV

“…1a). R403C, V446L, V447P and S456L did colocalize to a much lesser extent with LysoTracker compared with WT and rather showed an expression pattern similar to L106P or D362Y, reported previously to colocalize with the ER 35,36 . To quantify the relative degree of colocalization between LysoTracker and TRPML1 or the MLIV mutants, we calculated the respective Pearson Correlation Coefficients (Fig.…”

Loss of Lysosomal Ion Channel Transient Receptor Potential Channel Mucolipin-1 (TRPML1) Leads to Cathepsin B-dependent Apoptosis

“…Unlike most TRP channels, TRPML1 and its relatives TRPML2 and TRPML3 primarily reside in the membranes of the endocytic pathway (1)(2)(3)(4)(5)(6). TRPML1 is localized to later compartments along the endocytic pathway (late endosomes and lysosomes) by dileucine targeting sequences on its N and C termini (5,6) and has been shown to be an inwardly rectifying cation channel potentiated by low, typically lysosomal, levels of pH (7,8).…”