Cytoskeleton Side notes: Cytoskeleton Cytoskeleton is a cellular sca olding or a skeleton contained within the cytoplasm Present In all cells Cytoskeleton is a dynamic structure Functions: Maintains cell shape, protects the cell, enables cellular motion Plays important roles in intracellular transport and cellular division Three main types of filaments in cytoskeleton Intermediate filaments Provide mechanical strength Microtubules Determine the position of membrane-enclosed organelles Direct intracellular transport Actin filaments Determine the shape of the cell surface Necessary for cell movement Which filaments help to change the shape of a cell when it moves? 1. Intermediate filaments 2. Actin filaments 3. Microtubules Structures that cytoskeletal filaments can form Microtubules Mitotic spindle Cilia, flagella Bundles for transport along the axons Intermediate filaments Lining of the inner face of the nuclear envelope Cables in the cytosol Form tough appendages such as hair and fingernails Actin filaments A layer under the plasma membrane providing for strength and shape of the lipid bilayer Cell-surface projections: lamellipodia, filopodia for cell movement Contractile ring in cell division Muscles Bundles of stereocilia in the auditory cells, responsible for hearing Microvilli on the surface of intestinal epithelial cells Cytoskeletal filaments are dynamic Reorganization of the cytoskeleton during cell division Microtubules, which form mitotic spindle Actin filaments, which form contraceptive ring Organization of cytoskeleton in epithelial cells includes all three types of filaments Actin filaments form microvilli for absorption of nutrients Intermediate filaments provide connection to the basal lamina and between the cells Microtubules run vertically from the top to the bo om of the cell to direct newly synthesized components Subunits of the filaments Microtubules: tubulin subunits Actin filaments: actin subunits Cytoskeleton Side notes: Intermediate filaments: intermediate filament proteins such as keratin, destin, laminin, etc. Subunits assemble into protofilaments Protofilaments assemble into filaments Filaments have higher thermal stability than protofilaments Protofilaments are kept together with hydrophobic interactions and weak noncovalent bonds Polarity of the filaments Microtubules and actin filament subunits are not symmetrical As a result, filaments are also not symmetrical Filaments have plus and minus ends The polarity concept here has nothing to do with charges Intermediate filaments are composed of symmetrical monomers, which makes them non-polar A filament is most likely to disassemble 1. By subunits breaking Off at the end 2. By breaking in the middle Plus and minus ends Plus end is more dynamic: both adding and losing subunits is going faster at the plus end At the minus end, subunits are more stably connected The rates of addition and dissociation of subunits depends on the concentration of available subunits in the cytoplasm If there are a lot of subunits available, the filaments are more likely to grow The subunits may be present in the cytosol but be unavailable for forming filaments because they are in complexes with some blocking proteins Cells regulate availability of subunits. In response to a signal, blocking proteins can get phosphorylated, which changes their a inity to subunits Each type of filaments is constructed from smaller protein subunits Formation of polymers from subunits is strictly regulated inside the cell Cells reassemble the actin filaments in response to signals