Polymer-induced membrane contraction, phase separation, and fusion via Marangoni flow

Experiments have shown that the depletion of polymer in the region between two apposed (contacting or nearly contacting) bilayer membranes leads to fu sion. In this paper we show theoretically that the addition of nonadsorbing polymer in solution can promote lateral contraction and phase separation o f the lipids in the outer monolayers of the membranes exposed to the polyme r solution, i.e., outside the contact zone. This initial phase coexistence of higher- and lower-density lipid domains in the outer monolayer results i n surface tension gradients in the outer monolayer. Initially, the inner la yer lipids are not exposed to the polymer solution and remain in their orig inal "unstressed" state. The differential stresses on the bilayers give ris e to a Marangoni flow of lipid from the outer monolayers in the "contact zo ne" (where there is little polymer and hence a uniform phase) to the outer monolayers in the "reservoir" (where initially the surface tension gradient s are large due to the polymer-induced phase separation). As a result, the low-density domains of the outer monolayers in the contact zone expose thei r hydrophobic chains, and those of the inner monolayers, to the solvent and to each other across the narrow water gap, allowing fusion to occur via a hydrophobic interaction. More generally, this type of mechanism suggests th at fusion and other intermembrane interactions may be triggered by Marangon i flows induced by surface tension gradients that provide "action at a dist ance" far from the fusion or interaction zone.