Multifunctional transporter with an impact in cell morphology and differentiation (PubMed:20185830). In the presence of borate B(OH)4(-), acts as a voltage-dependent electrogenic Na(+)-coupled B(OH)4(-) cotransporter controlling boron homeostasis (By similarity). At early stages of stem cell differentiation, participates in synergy with ITGA5-ITGB1 and ITGAV-ITGB3 integrins and BMPR1A to promote cell adhesion and contractility that drives differentiation toward osteogenic commitment while inhibiting adipogenesis (PubMed:33247189). In the absence of B(OH)4(-), acts as a Na(+)-coupled OH(-) or H(+) permeable channel with implications in cellular redox balance. Regulates the oxidative stress response in corneal endothelium by enhancing antioxidant defenses and protecting cells from reactive oxygen species. In response to hypo-osmotic challenge, also acts as water permeable channel at the basolateral cell membrane of corneal endothelial cells and facilitates transendothelial fluid reabsorption in the aqueous humor. In the presence of ammonia, acts as an electrogenic NH3/H(+) cotransporter and may play a role in ammonia transport and reabsorption in renal Henle's loop epithelium (By similarity). 2 Na(+)(in) + tetrahydroxoborate(in) = 2 Na(+)(out) + tetrahydroxoborate(out) Homodimer. Expressed in the endothelial cells of the cornea. In the inner ear, is located in fibrocytes underlying the stria vascularis. In the kidney, is expressed in the thin descending limb of Henle loop. Glycosylated. Mutant mice show corneal defects characterized by progressive thickening of the stroma and descement membrane concomitant with increased sodium chloride concentration in the stroma (PubMed:20185830, PubMed:23813972). They show impaired urinary concentration, increased urinary volume, and increased urinary sodium loss in the kidney. They also show stress-induced morphological changes of fibrocytes of the inner ear resulting in deafness. Belongs to the anion exchanger (TC 2.A.31) family.