The kinetic mechanism of cation-catalyzed phosphatidylglycerol transbilayer migration implies close contact between vesicles as an intermediate state

Abstract

We have investigated variations in the rate of Mn2+-catalyzed phosphatidylglycerol transbilayer migration [Lentz, Madden, & Alford (1982) Biochemistry 21, 6799] with changes in phospholipid and cation concentration over more than a 100-fold range of both parameters. The slope of a double logarithmic plot of the rate of transbilayer lipid migration versus lipid concentration was 1.7, suggesting that lipid redistribution was dependent on vesicle aggregation or collision. A model involving transitory dimerization of vesicles was able to account for the concentration dependence of the transbilayer redistribution rate. The observed variation in rate with the logarithm of Mn2+ concentration was complex: linear above 0.4 microM (corresponding to roughly 2.5 Mn2+ per vesicle) but involving a steeper dependence on Mn2+ below 0.04 microM (roughly four vesicles per Mn2+). The rate of transbilayer redistribution increased substantially between 37 and 56 degrees C, yielding a nonlinear Arrhenius plot. There was no evidence of either fusion or lipid exchange between vesicles at the low concentrations of Mn2+ needed for transbilayer redistribution. The data are consistent with a model suggesting transitory "micro-domains" of a dehydrated, interbilayer complex as involved in the transition state and are inconsistent with a model involving an inverted micelle-type structure for the transition state.

Transbilayer Lipid Redistribution Accompanies Poly(ethylene glycol) Treatment of Model Membranes but Is Not Induced by Fusion

“…Our results clearly demonstrate that rupture is accompanied by enhanced NBD-PE flip (Figure 3 or 4). Indeed, flip can occur under conditions of extreme dehydration and membrane stress, even in the absence of fusion (Figure 3C;and Lentz et al, 1989). Our results also show that NBD-PE flip also accompanies fusion in the absence of vesicle rupture (Figures 3A,B, Figure 4A).…”

“…Intimate intermembrane contact appears necessary for fusion (Portis et al, 1979;Burgess et al, 1992). Mg 2+induced formation of a tight dehydrated complex between phosphatidylglycerol-containing membranes has been shown to mediate flip-flop of lipid in the absence of fusion (Lentz et al, 1989), even though lipid redistribution is normally very slow in pure lipid vesicles (Shaw et al, 1977). It might be expected that the close contact that results in mixing of lipids between apposed membrane leaflets (Lee & Lentz, 1997b) might also be sufficient to bring about transbilayer redistribution of lipid.…”

Interfacial Catalysis by Phosphoinositide 3'-Hydroxykinase

“…In general, any substance being evaluated for its effects on PI3K interfacial catalysis has the potential to affect the quality of the interface. Magnesium, for example, is required for PI3K utilization of ATP, but divalent cations are potent vesicle fusigens and can result in vesicle aggregation (Nir et al, 1983;Eastman et al, 1992;Lentz et al, 1989). Peptides are another class of molecules which are known to cause vesicle fusion and aggregation (Kubesch et al, 1987;Sundler & Bondeson, 1990), while detergents may inhibit PI3K by diluting the interfacial lipid substrate concentration and reducing the size of vesicle through the formation of micelles.…”

Evidence for membrane flow through pores in stacked phospholipid membranes

“…We also considered alternative mechanisms for the lipid flow from the distal membrane to the proximal membrane by direct lipid exchange, as described by Kunze et al 30 and others. 31,32 Such mechanisms comprise three slow transfer models: (i) monomer transfer, (ii) transient collision, and (iii) insertion (a variant of (ii)). These processes are far too slow for transporting lipids from bilayer to bilayer during the rapid rupture.…”