DNA – Membrane Interactions

We have developed a technique to study interactions between a fluid membrane and a single polymer, two soft objects subject to thermal fluctuations [1] [2]. We graft long lambda-phage DNAs by one or both ends on a flat substrate coated with streptavidin receptors, and induce adhesion of giant vesicles bearing biotin ligands in the membrane on this carpet. We showed that during the adhesion of the vesicle, the membrane has a steamroller effect on the end-grafted DNAs, which are stretched and confined in a frozen conformation. A quantitative analysis of fluorescence microscopy reveals the different interactions determining the confinement process – see Figure 1.

Figure 1. (a) Fluorescence image of single and double end-grafted DNAs, confined under the membrane of a giant unilamellar vesicle adhered to a bio-functional substrate. The DNA orientation results from the radial forces exerted by the membrane during the formation of the adhesive contact. Local fluorescence heterogeneity provides information about the DNA confinement tunnel, and a careful analysis of the loop shapes reveals the strength and direction of the forces acting on the DNA during the stretching and confinement process; (b, c) different observed configurations and model for explaining the formation of ’Eiffel Tower’ conformations.

We show that the macromolecules are trapped in a membrane ’tunnel’, defined by the multiple biotin- streptavidin links that have connected the bilayer to the substrate during the adhesion. Stretching of the DNA can be controlled by the amount of biotinilated phospholipids that regulates the kinetics of adhesion of the membrane. The analysis also reveals that there is a strong correlation between the density of fluorescence inhomogeneity along a string of DNA and the tortuosity of the tunnel. We interpret these inhomogeneities as evidence of the existence for self- entanglements, labile states that were trapped and frozen by the advancing membrane front [3].
In collaboration with Damien Baigl, ENS Paris, we were able to reversibly photo-induce collapsed and adhered states of the grafted DNAs using the photosensitive molecule AzoTAB [4].