CORRELATION BETWEEN BILAYER-LIPID DYNAMICS AND ACTIVITY OF THE DIGLUCOSYLDIACYLGLYCEROL SYNTHASE FROM ACHOLEPLASMA-LAIDLAWII MEMBRANES

In the single membrane of Acholeplasma laidlawii a specific glucosyltr ansferase synthesize the major, lamellar-forming lipid diglucosyldiacy lglycerol (DGlcDAG) from the major, nonlamellar-prone monoglucosyldiac ylglycerol (MGlcDAG). This is crucial for the maintenance of phase equ ilibria close to a bilayer-nonbilayer transition and a nearly constant spontaneous curvature in the membrane lipid bilayer, Acyl chain order is also affected, but not kept constant. Phosphatidylglycerol (PG) is an essential activator, needed in substantial amounts by the DGlcDAG synthase, and likely to affect bilayer properties, A potential connect ion was investigated between the (i) lateral diffusion, (ii) domain fo rmation of the PG activator, and (iii) bilayer chain ordering (i.e., t he hydrocarbon free volume), revealed in unilamellar liposomes by lipi d probes containing one pr two (fluorescent) pyrene acyl chains, and ( iv) activity of the DGlcDAG synthase. Different activator, nonbilayer perturbant, and bilayer matrix conditions were employed. Diffusion of PG was substantially slower in a DGlcDAG compared to a phosphatidylcho line (PC) matrix with 18:1c chains but increased with the PG content i n both. No obvious correlation between diffusion and enzyme activity, and no local concentration of PG as a function of chain ordering or cu rvature, was detected. However, an enrichment of PG activator into dom ains could be induced by a chain length mismatch between 18:1c-PG and 14:1c-PC (but not 22:1c-PC), even at small PG fractions. Patching was sufficient to stimulate enzyme activity 4-fold in relation to the acti vities normally valid al low PG concentrations. Chain order was substa ntially lower (i.e., free volumes larger) in bilayers of DGlcDAG than in bilayers of PC and increased in an additive fashion in both by the content of especially the nonbilayer-prone 1,3-18:1c-DAG but also by P G. At physiological concentrations of PG in DGlcDAG bilayers (approxim ate to 20%) a good correlation was evident between increased DAG conte nt and chain ordering and strongly enhanced enzyme activities, with ma xima close to a bilayer-nonbilayer transition. It is concluded that re gulation of packing conditions in A. laidlawii membranes by the DGlcDA G synthase seems to be governed not by the absolute extent of chain or der but more by the spontaneous curvature within a certain range of co nditions. Domain formation of the essential PG activator due to bilaye r conditions is a second mechanism, potentially overriding the curvatu re effects.