Coexistence of long and short DNA constructs within adhesion plaques

Abstract

Adhesion domains forming at the membrane interfaces between two cells or a cell and the extracellular matrix commonly involve multiple proteins bridges. However, the physical mechanisms governing the domain structures are not yet fully resolved. Here we present a joint experimental and theoretical study of a mimetic model-system, based on giant unilammelar vesicles interacting with supported lipid bilayers, with which the underlying physical effects can be clearly identified. In our case, adhesion is induced by simultaneous action of DNA linkers with two different lengths. We study the organization of bridges into domains as a function of relative fraction of long and short DNA constructs. Irrespective of the composition, we systematically find adhesion domains with coexisting DNA bridge types, despite their relative differences in length of 9 nm. However, at short length scales, below the optical resolution of the microscope, simulations suggest the formation of nanodomains by the minority fraction. The nano-aggregation is more significant for long bridges, which are also more stable, even though the enthalpy of membrane insertion is the same for both species.Cell adhesion is a fundamental biological process that relies on the formation of complex macromolecular assemblies, involving a number of proteins with different binding affinities, elastic properties and lengths 1 . These proteins may mediate adhesion in either a sequential manner, as it is the case of weak selectin adhesion preceding strong integrin adhesion in rolling leukocytes 2-4 , or simultaneously, for example during the formation of immune synapses. In this case T-cell receptors and integrins segregate 5,6 , presumably because of the differences in the length of their respective adhesive constructs 7 . However, the full understanding of the length-induced separation is still missing.One proven approach to study these mechanisms is using cell mimetic systems consisting of functionalized giant unilamelar vesicles (GUVs) adhering to supported lipid bilayers (SLB). So far, most mimetic adhesion systems involved only one protein pair, or + contributed equally mixtures between binders and repellers 8,9 , while the design of systems consisting of more than one binding pair is still challenging, and an active field of studies 10 .

Molecular Biomechanics Controls Protein Mixing and Segregation in Adherent Membranes

“…The current implementation of the code can account for ligands, receptors, repellers, intermittent bridges (which can swap between membranes) and other protein types [ 23 ]. Besides trans interactions, it is also possible to account for direct lateral cis interactions, and several protein types (mobile or arrested binders, tethers, repellers), however, at the expense of additional parametrisation.…”

“…On the other hand, for smaller affinities and smaller separations, adhesion proceeds through the formation of numerous and small, but mixed clusters that quickly merge (top right part of the phase diagram). The composition of the mixed state depends on the relative densities of the two ligands and receptors, as discussed recently [ 23 ].…”