Targeting Sequence and Function-Dependence of Subcellular Localization of Transient Receptor Potential Mucolipin Channels

Abstract

Both of TRPML1 and TRPML3 are members of the mucolipin subfamily of Transient Receptor Potential (TRP) channels. They have been implicated in endolysosomal functions such as divalent cation release, luminal pH regulation and vesicle trafficking. Interestingly, whereas TRPML1 is almost extensively localized in lysosomes, TRPML3 is typically found in both lysosomes and the plasma membrane (PM). It has been shown that TRPML3 localization depends on TRPML1, but the mechanism remains unknown. Recently, a number of TRPML1 mutants have been reported to be either retained in the endoplasmic reticulum (ER) or preferentially targeted the PM. Coexpressing these mutants with either wild type TRPML3 or a non-functional ER-retained mutant, TRPML3-KK, we examined how differentially targeted TRPML1 proteins affect TRPML3 localization. We show that while the wild type TRPML1 decreased the PM targeting of TRPML3, the PM-targeted TRPML1 mutants did not enhance PM localization of TRPML3. Interestingly, not only the wild type TRPML1 brought TRPML3-KK to lysosomes, but also TRPML3 brought the ER-retained TRPML1-KK to these acidic organelles. Moreover, coexpression of TRPML-KK not only reduced TRPML3-mediated Ca2+ response to a TRPML agonist, but also nearly abolished PM localization of TRPML3. We conclude that while the lysosome targeting sequence(s) is important for sorting TRPML channels out of ER-Golgi network, the ion-conducting function may be critical for PM trafficking of these endolysosomal channels.