Localization and functional requirement of yeast Na ⁺/H ⁺ exchanger, Nhx1p, in the endocytic and protein recycling pathway

Acidification of the lumen of intracellular organelles is important for post-transcriptional processing, endosomal maturation, receptor recycling, and vesicle trafficking, being regulated by an intricate balance between H ⁺ influx through vacuolar-type H ⁺-ATPase and efflux through ion channels and transporters, such as the Na ⁺/H ⁺ exchanger (NHE). The eukaryotic NHE family comprises two major subgroups, one residing in the plasma membrane and the other in intracellular organelles. While mammalian intracellular NHE isoforms are localized to various organelles, including the mid-trans-Golgi compartments, early and late endosomes, and recycling endosomes, Nhx1p, the sole NHE in yeast, has been reported to be localized predominantly to the late endosomal/prevacuolar compartment. Here, using live cell imaging, we demonstrated that Nhx1p is localized to the trans-Golgi network compartments, late endosomes, and recycling endosomes, similar to mammalian intracellular NHE isoforms. Loss of Nhx1p led to accumulation of components of the retromer and endosomal sorting complex required for transport complexes, but not trans-Golgi compartments, in aberrant prevacuolar compartments. Importantly, Nhx1p was also required for recycling of the plasma membrane vesicle SNAP receptor Snc1p. These observations suggest that Nhx1p plays an important role in regulation of the luminal pH of various intracellular organelles, and that this regulation is critical for the protein recycling pathway as well as the endocytic pathway.