Ventral Stress Fibers Induce Plasma Membrane Deformation in Human Fibroblasts

Interactions between the actin cytoskeleton and the plasma membrane are essential for many eukaryotic cellular processes. During these processes, actin fibers deform the cell membrane outward by applying forces parallel to the fibers major axis (as in migration) or they deform the membrane inward by applying forces perpendicular to the fibers major axis (as during cytokinesis). Here we describe a novel actin-membrane interaction in human dermal myofibroblasts. When labeled with a cytosolic fluorophore, the myofibroblasts developed prominent fluorescent structures on the ventral side of the cell. These structures are present in the cell membrane and colocalize with ventral actin stress fibers, suggesting that the fibers bend the membrane to form a "cytosolic pocket" for the fluorophores to flow into, creating the observed structures. The existence of this pocket was confirmed by transmission electron microscopy. Dissolving the stress fibers, inhibiting fiber protein binding, or inhibiting myosin II binding of actin removed the observed structures. However, decreasing cellular contractility did not remove the structures. Taken together, our results illustrate a novel actin-membrane bending topology where the membrane is deformed outwards rather than being pinched inwards, resembling the topological inverse of cytokinesis.