Cytoskeleton & Motor Proteins

Cytoskeletal Structure

The cytoskeleton in the eukaryotic cell is made up of three kinds of protein filaments:

• Actin filaments (also called microfilaments): Monomers of the protein actin polymerize to form long, thin fibers that are about 8 nm in diameter. They provide mechanical strength to the cell, link transmembrane and cytoplasmic proteins, anchor centrosomes during mitosis, generate locomotion in cells and interact with myosin to provide the force of muscular contraction
• Intermediate filaments: Cytoplasmic fibers averaging 10 nm in diameter. There are several types each constructed from one or more protein (e.g. keratins, nuclear lamins, neurofilaments, vimentins). All types of intermediate filaments provide a supporting framework within the cell
• Microtubules: Straight, hollow cylinders averaging 25 nm in diameter, built of α-tubulin and β-tubulin dimers. They participate in a wide variety of cellular activities with most involving motion. Motion is provided by protein 'motors' that use the energy of ATP hydrolysis to move along the microtubule

Motor Proteins

Motor proteins are the driving force behind muscle contraction and are responsible for the active transport of most proteins and vesicles in the cytoplasm. They are a class of molecular motors that are able to move along the surface of a suitable substrate, powered by the hydrolysis of ATP. There are three superfamilies of cytoskeletal motor proteins. Myosin motors act upon actin filaments to generate cell surface contractions and other morphological changes, as well as vesicle motility, cytoplasmic streaming and muscle cell contraction. The kinesin and dynein microtubule based motor superfamilies move vesicles and organelles within cells, cause the beating of flagella and cilia, and act within the mitotic and meiotic spindles to segregate replicated chromosomes.