Energetics and dynamics of membrane necks in particle wrapping

Transport of microscopic objects across biological membranes usually involves membrane deformation to enclose the object followed by detachment of the engulfed particle. However, in artificial membranes, this last topological remodelling step is in many cases not spontaneous due to the elastic stability of the neck structure formed upon complete particle wrapping. In this work, we use optical trapping to induce the wrapping of a non-adhesive microsphere by the membrane of a giant lipid vesicle and investigate the energetics and dynamics of the resulting neck structure. We find that neck formation occurs as a result of membrane shape energy minimization under the application of external force. Remarkably, increasing membrane tension could reopen the neck and reverse the wrapping process. This process shows a clear hysteresis and a degree of reversibility. Neck cleavage and particle detachment into the vesicle's interior could not be triggered in the range of our optical forces. Systematic studies on the thermal dynamics of wrapped particles allowed to establish that diffusion properties of the system are in agreement with a coupling of the particle motion with the neck structure, modeled as a solid inclusion within the membrane. Interestingly, the wrapped particle dynamics exhibited a tension dependency, which can be described as the sum of several drag contributions.