<P> Eukaryotic cells contain three main kinds of cytoskeletal filaments: microfilaments, microtubules, and intermediate filaments . Each cytoskeletal filament type is formed by polymerization of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution . Microfilaments are polymers of the protein actin and are 7 nm in diameter . Microtubules are composed of tubulin and are 25 nm in diameter . Intermediate filaments are composed of various proteins, depending on the type of cell in which they are found; they are normally 8 - 12 nm in diameter . The cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol, it adds to the level of macromolecular crowding in this compartment . Cytoskeletal elements interact extensively and intimately with cellular membranes . </P> <P> Neurodegenerative disorders have recently become more understood in context to what parts of the cell they affect . Diseases, such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis (ALS) have had breakthrough research that supports the conclusion that neurodegenerative diseases affect the cytoskeleton . Parkinson's disease is a condition that causes the degradation of neurons, resulting in tremors, rigidity, and other non-motor symptoms . Research has found evidence that microtubule assembly and stability in the cytoskeleton is compromised causing the neurons to degrade over time . Alzheimer's disease is much like Parkinson's in that it is also a neurodegenerative disease that affects the cytoskeleton . Tau proteins, which stabilize microtubules, malfunction in patient's affected by Alzheimers, causing pathology with the cytoskeleton . Huntington's disease has also been found to affect the cytoskeleton of cells by excess glutamine in the Huntington protein, which is involved with linking vesicles to the cytoskeleton . An error with this protein is proposed to be a factor in the development of the disease . A fourth neurodegenerative disorder is amyotrophic lateral sclerosis, or ALS, which causes a loss of movement by the degradation of motor neurons from defects of the cytoskeleton . </P> <P> A number of small - molecule cytoskeletal drugs have been discovered that interact with actin and microtubules . These compounds have proven useful in studying the cytoskeleton and several have clinical applications . </P> <P> All filaments interact with accessory proteins that regulate and link the filaments to other cell compounds and each other . The accessory proteins are essential for controlled assembly of cytoskeletal filaments in particular locations, and they include motor proteins . </P>

Which of the following is not a part of cytoskeletons