Electroneutral exchange of protons for Na(+) and K(+) across the early and recycling endosome membranes. Contributes to calcium homeostasis. Homodimer (in vitro). Is present in the recycling compartments including early and recycling endosomes, and only appears transiently on the plasma membrane. Ubiquitous; but is most abundant in mitochondrion-rich tissues such as brain, skeletal muscle and heart. Ubiquitinated (in vitro). The disease is caused by variants affecting the gene represented in this entry. Belongs to the monovalent cation:proton antiporter 1 (CPA1) transporter (TC 2.A.36) family. Was originally (PubMed:9507001) identified as a mitochondrial inner membrane protein, but was later shown to be localized in early and recycling endosomes and not mitochondria (PubMed:11940519).