Part 4 final exam Molecular Motors and Cell Behavior Myosins motor proteins associated with F actin o Myosins are a huge family of motor proteins that bind to microfilaments o Family can be divided into conventional type II myosins and 14 types of un conventional myosins Myosin II structure o Heteromer with 6 polypeptide chains one pair of heavy and two pairs of light chains Each heavy chain contains An S1 head with ATP ase activity on N terminus A neck region A coiled coil tail o Dimerization occurs when the 2 alpha helices of the heavy chains wrap around each other to form a coiled coil driven by the association of regularly spaced hydrophobic amino acids o The coiled coil arrangement makes an extended rod in solution tail o The heavy chain can be split into isolated soluble S1 heads and coiled coil tails by mild proteolytic enzymes o Isolated S1 heads bind to actin filaments and if the heads are attached to a coverslip and provided w ATP they move filaments across the coverslip Therefore they contain all the necessary motile machinery o Myosin II molecules can associate into filaments o These filaments are highly stable in muscle cells where bipolar myosin filaments form a basic structural unit of the contractile machinery o In non muscle cells myosin II filaments are only formed transiently as needed to move elements of the cytoplasm around o o Myosin II can move actin filaments by attaching to actin filaments moving power stroke and then detaching Cellular motility mediated by actin and myosin examples o Skeletal muscle contraction in animals o Cytoplasmic streaming in many types of cells o Movement of cargo along actin filaments o Amoeboid movement of cell or crawling Cellular locomotion non muscle motility o Muscle contraction o Multinucleated cells form by fusion of many muscle cell precursors myoblasts o Activation of skeletal muscle contraction Action potentials are triggered by the nervous system within the muscle fiber The propagate along the muscle fibers plasma membrane and penetrate to the center of the fiber by following invaginations of the plasma membrane called t tubules T tubules come into close contact with a specialized form of smooth ER called the sarcoplasmic reticulum a calcium ion store o When the action potential reaches the junctions between t tubules and sarcoplasmic reticulum it triggers Ca release from the sarcoplasmic reticulum into the muscle cell s cytosol around the myofibrils o Released Ca2 enter the muscle fiber cytosol and bind to troponin on the actin filaments of the myofibrils o Tropinin changes conformation and pushes tropomyosin out of a groove in the actin filament allowing myosin head to bind to actin and move it producing muscle contraction o Myosin I only has a single head unconventional myosin o It can bind to and push against actin filaments but single molecules of Myosin I find it difficult to walk along actin filaments since with only one head it must hop and stand a chance of detaching during the hops Cytoplasmic streaming o Some unconventional myosins are attached to the plasma membrane and can be used to drag microfilaments and attached cytoplasm across the inner surface of the membrane Cargo Movement o Some unconventional myosins can bind to various types of cargo and can move them along microfilament tracks o Most move cargo towards the end of microvilli E g localization of mRNA to growing tips of budding yeast cells Amoeboid movement of Fibroblast and keratocytes crawling o Actin and myosin play a leading role in many forms of non muscle motility such as the crawling movement of some cells across surfaces o Motile cells like human fibroblastsor fish keratinocytes pigmented skin cells as well as axons developing neurons have a specialized leading edge called the lamellipodium or nerve growth cone in neurons o These cells axons are capable of moving growing towards a chemical The leading edge of these cells is filled with a network rich in actin signal microfilaments o Fibroblast locomotion Reception of a chemical signal molecule by a receptor protein in the plasma membrane of the leading edge of the cell triggers the assembly of an actin cytoskeleton via a protein complex called the Arp2 3 The assembly of the actin network at the leading edge of the lamellipodium pushes it against the plasma membrane and extends it forward protrusion Contraction at the rear of the cell propels the cell forward to relax some of the tension traction New focal contacts are made at the front onto the underlying surface in the new areas over which the plasma membrane extends The actin network at the trailing edge of the lamellipodium is disassembled by an enzyme cofilin retracting the cytoplasm at the trailing edge allowing the lamellipodium to move forward a form of actin filament treadmilling The lamellipodium drags the rest of the cell after it enabled by detachment of the cell from substrate at the trailing edge and flow of cytoplasm from the trailing edge towards the lamellipodium This flow of cytoplasm cytoplasmic streaming and is mediated by myosin motor proteins this is common on many other types of cells moves cytoplasm around even non motile cells Microtubule based motility and motor proteins o Vesicular transport the example of fast axonal transport in neurons Proteins peptides are synthesized in the cell bodies of nerve cells close to the nucleus but are requires throughout the nerve cell Most of these materials are compartmentalized into vesicles in the ER and Golgi complex of the cell body which are then transported down the axon along microtubule tracks Other vesicles are moving in the opposite direction carrying waste materials and regulatory factors back from nerve terminals to the cell body The fast axonal transport can proceed at rates up to 5 microns per second 400 mm per day This transport is mediated by motor proteins that move along the microtubules it requires ATP o A sensory nerve cell o Kinesins microtubule associated motor protein The 1st kinesin was isolated as a motor protein responsible for moving vesicles and organelles along nerve axons from the cell body to the synaptic terminals Composed of 2 light chains and 2 heavy chains Heavy chains entwined to create a stalk region made of coiled helices Two globular heads of the heavy chains have ATP binding sites The heads can bind to microtubules initiating ATP ase activity and movement of kinesin molecule and cargo along the microtubule Movement of kinesin only occurs
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