Membrane-mediated interactions

Interactions stemming from lipid membrane deformations

Lipid membrane deforming objects, such as proteins, can interact through the membrane curvature they induce. These interactions have been predicted by theory and studied by simulations, however, the governing equations are highlgy involved and quantitative experiments are difficult due to the small size and complexity of proteins.

In the Kraft lab we therefore use a model system that consists of giant unilamellar vesicles and membrane-deforming colloidal particles to quantitatively study the interactions. Studying just two fully-wrapped colloidal spheres and carefully excluding other interactions, we could measure that the interaction induced by the membrane deformation is positive, a few kT strong, and extends over a range of about 2 particle diameters.

Extrapolating from these measurements towards three interacting particles, however, is not straightforward as the interactions have been predicted to be non-additive. Measuring the interplay between three fully-wrapped spheres we found two preferred states: a linear arrangement and an equilateral triangle.
A mixture of wrapped and unwrapped particles follows a variety of assembly pathways on the membrane and leads to aggregates.

  • C. van der Wel, A. Vahid, A. Šarić, T. Idema, D. Heinrich, D.J. Kraft, Lipid membrane-mediated attraction between curvature inducing objects, Scientific Reports, 6, 32825 (2016)
  • C. van der Wel, D. Heinrich, D.J. Kraft, Microparticle assembly pathways on lipid membranes, Biophysical Journal, 113 (5), 1037-1046 (2017)
  • A. Azadbakht, B. Meadowcroft, J. Majek, A. Saric, D.J. Kraft, Non-additivity in interactions between three membrane-wrapped colloidal spheres, Biophysical Journal, 123(3), 307-316 (2024)

Lipid membrane wrapping

Endocytosis is a key cellular process involved in the uptake of nutrients, pathogens or the diagnosis and therapy of diseases. Most studies have focused on spherical objects, whereas biologically relevant shapes can be highly anisotropic. We used an experimental model system based on Giant Unilamellar Vesicles (GUVs) and dumbbell-shaped colloidal particles to mimic and investigate the first stage of the passive endocytic process: engulfment of an anisotropic object by the membrane. Our model has specific ligand-receptor interactions realized by mobile receptors on the vesicles and immobile ligands on the particles. Through a series of experiments, theory and molecular dynamics simulations, we quantify the wrapping process of anisotropic dumbbells by GUVs and identify distinct stages of the wrapping pathway. We find that the strong curvature variation in the neck of the dumbbell as well as membrane tension are crucial in determining both the speed of wrapping and the final states.

  • A. Azadbakht, B. Meadowcroft, T. Varkevisser, A. Saric, D.J. Kraft, Wrapping pathways of anisotropic dumbbell particles by giant unilamellar vesicles, Nano Letters, 23(10), 4267-4273 (2023)