Inter-membrane adhesion mediated by mobile linkers: Effect of receptor shortage

Abstract

Giant unilamellar vesicles (GUVs) adhering to supported bilayers were used as a model system to mimic ligand-receptor mediated cell-cell adhesion. We present the effect of varying the concentration of receptors (neutravidin on the bilayer) and ligands (biotin on the vesicle) on GUV adhesion and the organization of receptors in the adhesion zone. At high concentrations of both ligands and receptors, the adhesion is strong, all the available membrane is adhered and receptors are accumulated under the adhered membrane up to the geometrical limit of close packing. At low concentrations of receptors (<0.5%), and an arbitrary concentration of ligands ($0.1%), adhesion does not proceed to completion: the membrane is only partially bound and parts of it still fluctuate. The receptors tend to accumulate under the adhered membrane but the filling is partial. Receptors get jammed and form clusters with fractal like shapes along the rim of the adhered vesicle in such a way that the annular cluster prevents further filling of the adhesion disc. We characterize the filling in terms of a compaction factor and the final concentration. Interestingly, the closing of the ring of jammed clusters switches the interior of the adhesion disc from one thermodynamic ensemble to another. In the new ensemble the receptors sealed within the adhesion disc are mobile but their number is fixed. Under such conditions, the usually strong neutravidin/biotin bond is weak. The incomplete adhesion state can be attributed to a combination of the effects of diffusion, jamming and the competition of enthalpy and entropy on bond formation. The formation of jammed receptor clusters reported here represents a new mechanism that influences membrane adhesion.

Specific adhesion of membranes simultaneously supports dual heterogeneities in lipids and proteins

“…In contrast, the biotin-neutravidin concentrations in our GUVs and SLBs, i.e., 1% in each, place us well into the complete-adhesion regime described in Fenz et al 19 In this regime, binding proteins are homogeneously distributed in the adhesion zone. However, the GUVs in Fenz et al did not phase separate.…”

“…In our previous work we estimated that suppressing undulations could change the free energy of demixing by about 3 K. 11 This is inadequate to explain a shift in transition temperature of more than 14 K. However when membranes are bound via biotin-neutravidin, there is a debinding free energy cost of about 10 k B T per biotin-neutravidin bond. 19 We therefore infer that the ordered-phase domains we see in adhering regions may or may not be in thermodynamic equilibrium.…”

“…Those researchers mapped out how the states of protein distribution depend on the density of binders in the GUV and in the SLB and found that at high binder concentrations in the SLB, i.e., 0.5% or greater, the adhesion was complete and the adhesion zones were homogeneously dense in binding proteins. 19 However at low binder concentrations in the SLB, i.e., less than 0.5%, adhesion was incomplete and binding proteins were heterogeneously distributed in the adhesion zones. This was found to result in an annulus of high protein density surrounding a protein-free region (see Fig.…”

“…2 and 3 of Fenz et al). 19 This annulus was attributed to jamming of the proteins; jamming is a non-equilibrium state.…”

“…11,14 We have previously shown that non-specific membrane adhesion can promote the formation of an ordered lipid phase. 11 Traditionally, specific adhesion of membranes via protein-like binders [15][16][17][18][19] has been studied separately from lipid phase separation. Specific adhesion can produce heterogeneities in binding-protein distribution via equilibrium and kinetic mechanisms.…”