The possible role of surface charge in membrane organization in an acidophile

Thiobacillus ferrooxidans, an obligate acidophile, possesses an electron transport chain that uses oxidation of ferrous iron to generate proton motive force. The cells possess an efficient machinery for inter-conversion of the two components of the free energy, namely pH gradient and membrane potential. Incidentally, unlike most of the naturally occurring membranes, surface in T. ferrooxidans appears to be positively charged at or near its physiological pH. Independent estimate of such charge was obtained from binding studies with anionic optical and fluorescent probes. When the external pH is lowered below a critical value the inter-conversion of the two components of the proton motive force was no longer operative. A surface charge mediated phase separation of membrane is suggested as one of the possible mechanisms for the failure of such inter-conversion process.