Lecture 7: Motor Proteins

Motor proteins Motor proteins Bind to a polarized cytoskeletal filament Move along the filament Use energy from the ATP hydrolysis There are dozens of different motor proteins Structure of a motor protein: Head region (motor domain, which binds to a filament and hydrolyzes ATP) Tail region (binds to the cargo) Functions of motor proteins Can carry cargo: Membrane-enclosed organelles Golgi stacks Secretory vesicles RNA Cause cytoskeletal filaments to slide against each other Muscle contractions, ciliary beating, cell division Which type of cytoskeletal filaments is not associated with motor protein movements? • A. Microtubules • B. Intermediate filaments • C. Actin Groups of motor proteins Moving along actin filaments Myosins (mostly move to +, some to - ends) Moving along microtubules Kinesins (mostly move to + end) Dyneins (move to - end) Structural and functional heterogeneity of myosins is determined mainly by different tail regions All myosins have similar motor domains Most of them move to the plus end of an actin Myosin V is associated with organelle and vesicle transport Myosin II is responsible for contractile activity in muscle and non-muscle cells Myosin V carries cargo along actin filaments In budding yeast, myosin V carries organelles, mRNA, secretory vesicles, ER towards the bud Myosin V carries cargo towards the + end It is always attached to actin with at least one head domain Myosin II Responsible for muscle contraction Formed from 2 heavy chains plus 2 light chains Heavy chains have globular head domains (ATPases) and extended coil-coil domain for dimerization Myosin II bipolar thick filament (frog muscle) Several hundred of myosin II molecules form bundles through their long coil-coil domains Myosin heads are sticking out of the thick filaments Myosin II is a ______ • A. Cytoskeletal filament • B. Motor protein The role of Myosin II in the skeletal muscle contraction Each myosin head binds and hydrolyzes ATP The energy of the ATP hydrolysis is used to move the myosin heads toward the plus end of the actin filament This results in sliding of actin filaments relative to myosin Myosin movement Myosin head attached to actin (rigor configuration) ATP binds to myosin; myosin head releases the actin ATP hydrolysis; the myosin head swings to the right (cocked conformation) Myosin head binds again; phosphate releases (force-generating step, power stroke) ADP released; myosin head returns to original attached conformation Which step is associated with the physical movement of myosin along actin (power stroke)? • A. ATP hydrolysis • B. Release of phosphate Muscle contraction A muscle cell is a muscle fiber. It formed by fusion of many cells and has many nuclei Muscle fiber consists of many myofibrils Myofibrils = cylindrical structures that span entire length of a muscle cell Consist of long repeated chains of contractile elements called sarcomeres which give it striated appearance Skeletal muscle Each sarcomere has a dark band and a half of a light band from each side Dark bands are mostly composed of thick filaments of myosin II Light bands are mostly composed of actin The Z-disk is a place of attachment of actin filaments plus ends When sarcomere contracts actin and myosin start to overlap more The mechanism of muscle contraction During muscle contraction myosin heads crawl to the actin filaments plus ends Actin and myosin filaments slide along each other The length of each sarcomere shortens Protein titin acts as a molecular spring, which allows the muscle fiber to return to its initial state Which of the following is responsible for dark bands appearance? • A. Actin • B. Myosin Muscle contraction is triggered by Ca2+ Low Ca2+ (resting muscle) In a resting muscle, protein complex troponin pulls tropomyosin from its normal binding groove into a position that interferes with binding of myosin heads High Ca2+ (contraction) In a contracting muscle, raising calcium concentration causes troponin to release tropomyosin. Tropomyosin slides into a position that does not interfere with myosin binding Myosin heads move along actin filament and muscle contracts How is the calcium concentration raised? Calcium is stored in the sarcoplasmic reticulum, which is a special web-like sheath of modified endoplasmic reticulum that surrounds each myofibril What is the signal for Ca2+ release Signal from nerve cell reaches the special T tubules that surround each myofibril. The Ca2+ channels in T tubules open and release some Ca2+, which binds to receptors on SR, which leads to Ca2+ massive release from SR What happens first? • A. Release of calcium from the T-tubules • B. Release of calcium from the sarcoplasmic reticulum The signaling pathway that triggers muscles contraction A motor neuron transmits an action potential to a muscle cell Voltage-dependent calcium channels open and Ca2+ ions flow into the motor neurons cytosol This influx of Ca2+ triggers a release of acetylcholine into the synaptic cleft Acetylcholine binds to the nicotinic acetylcholine receptors at the muscle cell The receptors are ligand-gated ion channels, and when bound by acetylcholine, they open, allowing sodium to flow in and potassium ions to flow out the muscles cytosol This produces a local depolarization of the muscle cell This depolarization spreads across the surface of the muscle fiber into T tubules This elicits the release of calcium from the sarcoplasmic reticulum, thus initiating muscle contraction