Ligand binding asymmetry and lipid fluidity changes in inside-out and right side-out plasma membrane vesicles

Abstract

A number of studies have suggested that some biological membrane functions might be correlated with the physical state of the phospholipids in these membranes [l-4] . Breaks in the Arrhenius plots of enzymatic or transport activities have indicated a possible relationship between phase transitions in the phospholipids [l-3] and these activities. Recently it has been shown by ‘H NMR and X-ray diffraction that in sarcopIasmic reticulum membranes no order + disorder type of transitions occur in the temperature range l-40°C, but breaks in the Arrhenius plots for the rates of Ca2+ transport and Ca” activated ATPase were observed between 15°C and 25°C [4] . This observation indicates a possible association between the character of the phospholipid segmental motion and the functional parameters of the membranes. Binding of Con A on right side-out (RSO) plasma membrane vesicles was shown to affect the activities of 5’nucleotidase (EC 3.1.3.5) and of (Na’ t K+) stimulated Mg2’-ATPase (EC 3.6.1.3), while it had no effect on the inside-out (IO) vesicles of the same plasma membranes. In contrast, a peptidoglycan-like adjuvant was effective in modulating the enzymes, only on the IO vesicles [5] . It is the purpose of this letter to show that binding of these ligands on the reactive membrane face affect the membrane phospholipid fluidity.