MEMBRANE LIPID ALKYL CHAIN MOTIONAL DYNAMICS IS CONSERVED IN SARCINA VENTRICULI DESPITE PH-INDUCED ADAPTATIVE STRUCTURAL MODIFICATIONS INCLUDING ALKYL CHAIN TAIL TO TAIL COUPLING

摘要

Sarcina ventriculi, an anaerobic Gram-positive bacterium, adapts to increasing temperature, the presence of organic solvents, or the lowering of the pH of its growth medium by joining the tails of membrane lipids from opposite sides of the bilayer, forming transmembrane, bifunctional fatty acid species. Since this is done to offset the increase in membrane mobility caused by these perturbations, it is of interest to determine whether the motional (dynamic) properties of membrane lipid alkyl chains are conserved. In this study, conservation of the motional time scales of the alkyl chains of total membrane lipids from Sarcina ventriculi cells grown at different pH values was demonstrated using proton nuclear magnetic resonance (NMR) spectroscopy. The NMR longitudinal relaxation times (T-1) of the protons in the bulk methylene groups were measured for lipids from cells grown at pH 3.0 and 7.0. These measurements indicated that the temperature profile of the T-1 relaxation behavior for the methylene protons from these two different preparations was the same. Analysis of the data from T-1 measurements indicated that the thermal barrier for relaxation is the same in both lipid systems. This is only true if the pH of the sample on which the measurement is being made is adjusted to the same value as that at which the corresponding cells were cultured. It is clear from this latter observation that the state of protonation of the lipid head groups is a contributor to the overall motional freedom of the membrane lipid components. The correlation times (tau(c)) of characteristic lipid alkyl chain motion were estimated to be approximately 10(-10) s. This study reaffirms the principle of homeoviscous adaptability and indicates that, during adaptation, conservation of structural features is secondary in importance to conservation of motional dynamics.