Transport and regulatory characteristics of the yeast bicarbonate transporter homolog Bor1p

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Abstract
The functional properties of the Saccharomyces cerevisiae bicarbonate transporter homolog Bor1p (YNL275wp) were characterized by measuring boron (H3BO3), Na+, and Cl− fluxes. Neither Na+ nor Cl− appears to be a transported substrate for Bor1p. Uphill efflux of boron mediated by Bor1p was demonstrated directly by loading cells with boron and resuspending in a low-boron medium. Cells with intact BOR1, but not the deletant strain, transport boron outward until the intracellular concentration is sevenfold lower than that in the medium. Boron efflux through Bor1p is a saturable function of intracellular boron (apparent Km ∼1–2 mM). The extracellular pH dependences of boron distribution and efflux indicate that uphill efflux is driven by the inward H+ gradient. Addition of 30 mM HCO3− does not affect boron extrusion by Bor1p, indicating that HCO3− does not participate in Bor1p function. Functional Bor1p is present in cells grown in medium with no added boron, and overnight growth in 10 mM H3BO3 causes only a small increase in the levels of functional Bor1p and in BOR1 mRNA. The fact that Bor1p is expressed when there is no need for boron extrusion and is not strongly induced in the presence of growth-inhibitory boron concentrations is surprising if the main physiological function of yeast Bor1p is boron efflux. A possible role in vacuolar dynamics for Bor1p was recently reported by Decker and Wickner (10). Under the conditions used presently, there appears to be mildly abnormal vacuolar morphology in the deletant strain.