Protein-Mediated Beads-on-a-String Structure Formation Along Membrane Nanotubes in Live Cells

Abstract

It is now established that various types of cells form thin tunneling nanotubes which connect their plasma membranes and enable intercellular communication. These dynamic structures adopt their configuration and functionality through various constraints including membrane tension, lipid asymmetry and cytoskeletal forces. Using a combination of metabolically labeling and bioorthogonal click chemistry, we implement single-molecule tracking and dynamic super-resolution imaging of tagged glycans on the membrane of live cells. We have observed that membrane nanotubes can manifest stable, “beads-on-a-string” structures. However, the physical and biological mechanisms underlying these structures are not clear, raising the following fundamental questions: How does the spatial density and type of proteins on the surface relate to the characteristic shape and formation of these structures? What is the protein distribution associated with the membrane curvature of the beads and what are the mechanisms that lead to the often observed stability of these structures? To address some of these questions, we model membrane nanotubes using the modified version of the Helfrich energy including surface protein interactions and diffusion. We find that a localized protein distribution along the membrane surface creates regions of low tension, which are energetically favorable for bead formation. Furthermore, for a fixed membrane area, the beads grow and appear to coalesce as the protein density or areal coverage increases. Diffusive flux of proteins expands the protein surface coverage, resulting in larger beads. Our results predict that the curvature-gradient at the edges of the beads acts as diffusion traps, confining the protein and stabilizing the beads-on-a-string structures.