Lecture 6: Cytoskeleton

Cytoskeleton Cytoskeleton Cytoskeleton is a cellular scaffolding 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 Microtubule s Determine the position of membrane-enclosed organelles Direct intracellular transport Actin filaments Determine the shape of the cell surface Necessary for cell movement 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 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 Which filaments help to change the shape of a cell when it moves? • A. Intermediate filaments • B. Actin filaments • C. Microtubules 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 bottom of the cell to direct newly synthesized components Subunits of the filaments Microtubules: tubulin subunits Actin filaments: actin subunits Intermediate filaments: intermediate filament proteins such as keratin, desmin, laminin, etc. Subunits assemble into protofilaments Protofilaments assemble into filaments Polarity of the filaments Microtubules and actin filament subunits are not symmetrical As a result, filaments are also not symmetrical Filaments have plus and minis ends The polarity concept here has nothing to do with charges Intermediate filaments are composed of symmetrical monomers, which makes them non-polar 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 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 or subunits. In response to a signal, blocking proteins can get phosphorylated, which changes their affinity to subunits Actin subunits are bound to profilin or thymosin; tubulin subunits can be blocked by stathmin 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 A filaments is most likely to disassemble • A. By breaking subunits the end • B. By breaking in the middle Actin filaments Actin subunit is an actin monomer Each actin subunit has a binding site for ATP Actin subunits assemble head to tail Actin filaments are polar (plus end is different from minus) Actin filaments assemble from two parallel protofilaments that are twisted around each other in a righthanded helix Actin filament ps are relatively flexible and easier to bend than microtubules ATP/GTP hydrolysis Free floating, cytoplasmic actin subunits have ATP; tubulin subunits have GTP Upon addition to filament, ATP/GTP is hydrolyzed to an ADP/GDP The hydrolysis is associated with a conformational change Subunits with ADP/GDP are easier to dissociate Treadmilling At some concentration of free subunits, the plus end grows and the minus end shrinks at the same rate The length of the filament stays the same This is called treadmilling Treadmilling can occur for both actin filaments and microtubules What happens to a filament during treadmilling? • A. It dissociates • B. It grows • C. It moves Actin filaments nucleation (starting a new filament from subunits) Occurs at or near the plasma membrane The highest density of actin is at the cell periphery The layer beneath the cell membrane is called the cell cortex The actin filaments in the cell cortex determine the shape and movement of the cell surface ARP and formins promote actin nucleation External signals regulate actin nucleation Nucleation of actin is catalyzed by two different types of regulated factors: the ARP complex (Actin related proteins) and the formins ARPs mostly catalyze the formation of branched structures, while formins catalyze the straight, nonbranched actin filaments, which can form parallel bundles ARPs ARPs are 45% identical to actin ARP complex nucleates actin at the minus end, allowing plus end to grow Formins can stabilize actin filaments by forming a ring over the plus end Formin travels with the elongating plus end Many formins are indirectly connected to the cell plasma membrane Some formins have whiskers that bind profilin in complexes with actin subunits. This promotes addition of actin subunits to the filament Which of the following helps to organize the contractile ring in mitosis? • A. ARPs • B. Formins Proteins that interact with filament ends In some case, the actin filaments should be stabilized from degradation They are stabilized by binding capping proteins at the end, which does not allow it to shrink or grow ARP complex and gamma-TuRC can serve as capping proteins for minus ends In muscle cells the actin filaments are long-lived and bound to capping proteins from both plus and minus ends