Cell Biology

Microfilaments, Myosin, and Intermediate Filaments in Cell Biology

• Microfilaments are involved in various cellular functions such as cytoplasmic streaming, amoeboid motion, and cell division.
• They can form different structures within the cell, including labile structures like cell cortex and stress fibers, and stable structures like microvillus and sarcomer.
• Actin, the protein component of microfilaments, has a diameter of 7nm and constitutes 5-10% of the total cellular protein.
• Actin is a globular protein that polymerizes to form filamentous actin, and it has a polarized structure with both positive and negative ends.
• Factors affecting microfilament formation include fungus toxins like cytochalasin B and amanita phalloidin, as well as proteins like profilin and thymosin β4 that control actin elongation.
• Capping proteins and drugs can also inhibit actin formation, and actin-binding proteins like fimbrin, filamin, and spectrin play crucial roles in organizing actin filaments.
• Myosin is a motor protein that converts chemical energy from ATP into mechanical energy and is involved in functions such as muscular contraction and cytoplasmic streaming.
• Myosin II forms thick filaments and is essential for cytokinesis in animal cells, providing the force for the contractile ring during cell division.
• Actin filaments provide mechanical strength to the cell, link transmembrane proteins to cytoplasmic proteins, and generate locomotion in cells such as white blood cells and amoeba.
• Intermediate filaments are the third major filament system in eukaryotic cells, resembling cables in structure and providing tensile strength, as seen in hair and nails.
• Intermediate filaments are biochemically heterogeneous, have great tensile strength, lack intrinsic polarity, and do not have known motors that use them as tracks.
• They help maintain cell shape, are durable, and may help maintain organelle position within the cell.