Energetics and dynamics of membrane necks in particle wrapping

Transport of microscopic particles across biological membranes involves large deformations of the membrane -first to wrap the particle, followed by the cleavage of the membrane neck, which leads to particle detachment. The stability and dynamics of these neck structures play a pivotal role in many membrane remodeling processes in biology and are expected to depend on both membrane and particle properties, as well as their interactions.

In this work, we use optical trapping and micropipette suction to induce the wrapping and unwrapping of a non-adhesive microsphere by the membrane of a giant lipid vesicle. The energetics and dynamics of the resulting neck structure are quantified probing different particle sizes and membrane tensions. In the range of the optical forces available in our setup, we also investigate the possibility to induce a particle detachment inside the vesicle. The dynamics of the membrane neck is also measured by tracking fully wrapped particles diffusing on the membrane surface.

We find that neck formation occurs as a result of membrane shape energy minimization under the application of an external force. Changing the direction of the applied force does not allow to observe the neck reopening. Remarkably, increasing membrane tension could reopen the neck and reverse the wrapping process, which shows a clear hysteresis and a degree of reversibility. Neck cleavage and particle detachment into the vesicle's interior could not be triggered. Diffusion measurements of wrapped particles reveal a coupling of the particle motion with the neck structure. Interestingly, the wrapped-particle dynamics exhibited a tension dependency, which we can describe as the sum of several drag contributions.