Figure 5.

Atomistic MD Simulations Capture Transbilayer Interdigitation of Long Acyl Chain Lipids (A) Equilibrium configurations of an asymmetric bilayer composed of POPC (gray), PSM (magenta), Chol (yellow), GPI (red or green), and PS (blue) embedded in water (cyan) from MDsimulations in the ld phase. Upper leaflet comprises 4% PSM, 4% Chol, 10 long saturated GPIC16:0/C16:0, and the rest POPC, whereas lower leaflet has 35% Chol, 25 long saturated PS C18:0/C18:0, and the rest POPC when PS is not constrained (right, Figure S6A) or when PS molecules are immobilized (left and middle) which, when zoomed (middle), shows bilayer registry and interdigitation of GPI and PS. Local region surrounding interdigitating PS and GPI consists of enhanced levels of PSM and Chol, resembling local lo-like nanodomain. (B) Extent of bilayer registry between GPI and PS measured by the transbilayer correlation coefficient Cul (Experimental Procedures), which takes values 1 (0) when bilayer registry is strong (weak). Data shown are for the same bilayer composition as in (A) above. Transbilayer coupling Cul is significant only when either PS or GPI are held in a cluster and immobilized. There is no transbilayer coupling and hence no registry when the GPI and PS are unconstrained. (C) Levels of cholesterol in upper/lower leaflets that are needed to obtain high transbilayer coupling Cul in the ld phase (white [high Cul] or gray [low Cul]). The cholesterol concentrations in the two leaflets are varied as shown (dots), the upper leaflet has 10 GPI, and the lower leaflet has 15 PS, with POPC contributing to the rest. PSM concentration at the upper leaflet is same as cholesterol. Red (blue) dots designate high Cul y1 (low, Culy0). (D) Coherence length of membrane component density and deuterium order parameter in the two leaflets at late times when PS is held immobilized, computed from the exponential decay of their profiles. Coherence length increases non-linearly with the number of immobilized PS. Composition in upper leaflet is 92% POPC, 4% PSM, 4% Chol, and 10 GPI and in lower leaflet is 65% POPC, 35% Chol with number of PS varying from 2 to 20. (E) Transbilayer coupling, Cul between GPI and PS is sensitive to lipid chain length and degree of saturation of acyl chains. Composition of upper leaflet is 33.3% PSM, 33.3% Chol, 10 GPI, and the rest POPC; the composition of the lower leaflet is 10% PS, 10% Chol, and the rest POPC. Chain length and degree of saturation of PS and GPI are varied as indicated. Strong transbilayer coupling and bilayer registry are obtained only when both PS and GPI have long saturated acyl chains.

Error bars represent SD. See also Figure S6.