Ca²⁺/H⁺ antiporter-like activity of human recombinant Bax inhibitor-1 reconstituted into liposomes

We investigated the functional activity of recombinant Bax inhibitor-1 reconstituted into liposomes. When proteoliposomes were suspended in acidic solutions, encapsulated Ca²⁺ was released from the membranes, as previously suggested [Kim HR, Lee GH, Ha KC, Ahn T, Moon JY, Lee BJ, Cho SG, Kim S, Seo YR, Shin YJ et al. (2008) J Biol Chem283, 15946-15955]. Concomitantly, proton ions were internalized when assayed using the time-dependent change in the fluorescence of the pH-sensitive dye oxonol V entrapped in the proteoliposomes. The influx of proton ions was confirmed by observing tritium accumulation in the membranes. However, the external acidity of the membranes per se did not induce proton ion influx without internalized Ca²⁺. These results suggest that reconstituted Bax inhibitor-1 has a Ca ²⁺/H⁺ antiporter-like activity.