  Living cells are capable of interacting with their environments in a variety
of ways, including cell signalling, adhesion, and directed motion. These
behaviours are often mediated by receptor molecules embedded in the cell
membrane, which bind specific ligands. Adhesion mediated by a large number of
weakly binding moieties - multivalent binding - is prevalent in a range of
active cellular processes, such as cell crawling and pathogen-host invasion. In
these circumstances, motion is often caused by gradients in ligand density,
which constitutes a simple example of chemotaxis. To unravel the biophysics of
chemotactic multivalent adhesion, we have designed an experimental system in
which artificial cell models based on lipid vesicles adhere to a substrate
through multivalent interactions, and perform chemotactic motion towards higher
ligand concentrations. Adhesion occurs via vesicle-anchored receptors and
substrate-anchored ligands, both consisting of synthetic DNA linkers that allow
precise control over binding strength. Experimental data, rationalised through
numerical and theoretical models, reveal that motion directionality is
correlated to both binding strength and vesicle size. Besides providing
insights into the biophysics of chemotactic multivalent adhesion, our results
highlight design rules applicable to the development of biomimetic motile
systems for synthetic biology and therapeutic applications.
