Protein filament

In biology, a protein filament is a long chain of protein monomers, such as those found in hair, muscle, or in flagella. Protein filaments form together to make the cytoskeleton of the cell. They are often bundled together to provide support, strength, and rigidity to the cell. When the filaments are packed up together, they are able to form three different cellular parts. The three major classes of protein filaments that make up the cytoskeleton include: actin filaments, microtubules and intermediate filaments. Microfilament Compared to the other parts of the cytoskeletons, the microfilaments contain the thinnest filaments, with a diameter of approximately 7 nm. Microfilaments are part of the cytoskeleton that are composed of protein called actin. Two strands of actin intertwined together form a filamentous structure allowing for the movement of motor proteins. Microfilaments can either occur in the monomeric G-actin or filamentous F-actin. Microfilaments are important when it comes to the overall organization of the plasma membrane. Actin filaments are considered to be both helical and flexible. They are composed of several actin monomers chained together which add to their flexibility. They are found in several places in the body including the microvilli, contractile rings, stress fibers, cellular cortex, etc. In a contractile ring, actin have the ability to help with cellular division while in the cellular cortex they can help with the structural integrity of the cell. Microfilament Polymerization Microfilament polymerization is divided into three steps. The nucleation step is the first step, and it is the rate limiting and slowest step of the process. Elongation is the next step in this process, and it is the rapid addition of actin monomers at both the plus and minus end of the microfilament. The final step is the steady state. At this state the addition of monomers will equal the subtraction of monomers causing the microfilament to no longer grow. This is known as the critical concentration of actin.