Characterization of the membranes of Thermoplasma acidophilum

Thermoplasma acidophilum grows optimally under aeration at 59 C and pH 2. Both intact cells and membranes disaggregate below pH 1 and above pH 5, producing no sedimentable particles. Increase in ionic strength at pH 5 or below results in cellular lysis and membrane disaggregation. Membranous components produced by lysis at alkaline pH reaggregate upon reduction of both pH and ionic strength. Osmotic environment plays little role in cellular stability. Membranes prepared by sonic lysis at pH 5 exhibit vesicular structures and are composed of multiple proteins. Although the amino acid composition of the membrane proteins is similar to other mycoplasmal membranes, the number of free amino and carboxyl groups is less than half of those in Acholeplasma. Reduction of the number of free carboxyl groups results in membrane stabilization over a wide range of pH. Increase in the number of free amino groups reverses the stability of membranes relative to pH. Acidophily in Thermoplasma can be related to a significant reduction in repulsing negative charges on the membrane proteins.