Magnetic manipulation of membrane molecule motion

Abstract

The plasma membrane is the organelle that encompasses the cell and represents the interface through which the cell interacts with its environment. As the membrane is fluid, membrane components can diffuse laterally in the plane of the membrane and their function often depends on their location and dynamics in the membrane. In polarized cells, where the plasma membrane is segregated into functional domains, the lateral diffusion must be restricted to ensure the distinct composition of the membrane domains. This is facilitated by diffusion barriers which impede the movement of molecules in the membrane but in many cases, it is unclear how the barriers are established, and which molecules are involved. At nanoscopic scales, the plasma membrane is thought to be compartmentalized by the underlying cytoskeleton and anchored transmembrane proteins (Picket and Fence model). To study these nanoscopic barriers, the random movements of membrane molecules are usually observed by microscopy, e.g. by single-particle tracking (SPT), and the movement pattern correlated with cellular structures such as the cortical cytoskeleton. However, to directly detect the presence of a physical barrier, a membrane probe could be steered against it to observe its interaction with the barrier. To do so, we developed a method using fluorescent magnetic nanoparticles (FMNPs) and magnetic tweezers. By coupling FMNPs to membrane molecules, we can follow their diffusion by SPT and direct their movement with magnetic tweezers. This way, we moved lipids through artificial lipid bilayers, and membrane proteins through the plasma membrane of living cells. We achieved SPT with ~10 nm localization precision and 5 ms time resolution, while dragging the molecules with magnetic forces of 1-10 fN. Pulling single membrane proteins over the cell surface, we were indeed able to detect obstacles to the protein motion and, with correlative superresolution imaging, localized them to the site of actin filaments underneath the plasma membrane. Our method hence enables the remote control of single molecule motion and the detection of diffusion barriers with high spatiotemporal resolution. On a broader perspective, the method can also be used to observe and perturb other coordinated activities of membrane molecules such as receptor signaling. We thus contribute a straightforward and versatile tool to investigate the dynamic organization of the plasma membrane at nanoscopic scales.