Polymerization of actin filaments: Actin polymerization plays a crucial role in membrane protrusion. Actin monomers are present in high concentrations near the cell's leading edge. These monomers polymerize to form actin filaments, which are elongated protein structures that push against the plasma membrane.
Membrane bending and protrusion: As actin filaments polymerize, they exert forces on the plasma membrane, causing it to bend and push outward. This leads to the formation of protrusions. Membrane curvature is regulated by proteins such as BAR (Bin/Amphiphysin/Rvs) domain proteins and other curvature-sensing proteins.
Adhesion to the extracellular matrix: To stabilize the protrusions and facilitate cell movement, transmembrane integrin proteins in the plasma membrane bind to ligands in the extracellular matrix (ECM). These interactions connect the actin cytoskeleton to the ECM, providing traction for cell movement.
Myosin motor activity: Myosin motor proteins, such as myosin II, interact with the actin filaments and move towards the cell's leading edge. This generates contractile forces that help drive the cell forward.
Membrane recycling: To accommodate the rapid extension of the plasma membrane during membrane protrusion, cells have various mechanisms for recycling membrane components. Endocytosis plays a crucial role in this process by retrieving excess membrane from the leading edge and recycling it back to the cell body.
It's important to note that the exact molecular mechanisms of membrane protrusion can vary between different cell types and different environmental conditions. Cells tightly regulate and coordinate these processes to control their shape, mobility, and adhesion in response to external cues and their specific functions.
