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Lecture 2 Muscle, myosin Outline: Brief overview of a long history Sarcomere structure and function Myosin Regulation of contraction Paper: A large protein required for sarcomere stability in flight muscle A brief history 1660 muscle dissected into fibers 1682 striations seen in skeletal muscle fibers 1700-1900 metabolism – lactic acid, heat production 1864 1st muscle prep – actomyosin – salt extraction of tissue 1939 ATPase activity of actomyosin demonstrated 1943 actin and myosin separated – different viscosity properties 1950s EM, X-ray diffraction structural studies 1954 sliding filament model proposed (H. Huxley) striated muscle multinucleate cells 10-100 mm thick up to 40 mm long light micrograph EM: Sarcomere = contractile unit banding pattern due to partial overlap of two types of filaments thick filaments = myosin thin filaments = actin EM cross section: hexagonal lattice of thin filaments surrounding thick filaments Question: How does muscle contract? Sliding Filament Model: thick and thin filaments slide past one another Evidence: 1) EM of sarcomeres at different stages during contractile process shows decreased width of banding pattern 2) both filament systems maintain constant length, region of overlap increases relaxed contracted How does sliding of filaments occur? 1960s Higher resolution EM - cross bridges, individual filaments myosin thick filaments: bipolar actin thin filaments: uniform polarity barbed (+) ends barbed (+) ends pointed (-) ends model by ~1970 molecular details still controversial 1980s-present reductionist approach in vitro reconstitutions: simplifed motility assays X-ray crystallography and EM reconstructions single molecule measurements primitive contractility assay superprecipitation: combine actomyosin with ATP in beaker, see what happens modern motility assay 1) Adsorb myosin molecules on glass coverslip in chamber 2) Perfuse in rhodamine-labeled actin filaments and ATP 3) Observe by fluorescence video microscopy - + + muscle myosin ~4.5 mm/sec Myosin - the most studied of all proteins (!?) large family of myosin-related proteins ~14 in human common features: one or two heavy chains and several light chains heavy chain: 1) large globular head: contains actin-binding and ATPase domains 2) a-helical neck region - binds light chains 3) tail domain - for oligomerization or cargo binding light chains: 1) calcium-binding proteins, sometimes calmodulin 2) regulate myosin activity myosin II vesicles, organelles muscle, stress fibers vesicles, organelles Myosin II mechanism ATPase activity stimulated by actin: from 4/hour to 20/second ATP binding, hydrolysis and dissociation of ADP-Pi produce a series of allosteric changes in myosin conformation Energy release is coupled to movement cross bridge cycle Myosin II crystal structure (S1 fragment) neck domain = lever arm superimpose structures in two different nucleotide states catalytic head Other evidence for lever arm model Spudich lab (1996): replace endogenous Dicteostelium myosin II gene with neck domain mutants - longer or shorter purify and measure velocity in motility assay velocity = step size/time bound to actin light chain binding sites WT (2) 1 0 3 motility assay 4 velocity 3 mm/sec 2 1 0 1 2 # of light chains 3 Current Issues/Questions How is the large conformational change of lever arm generated during phosphate release? How many steps are taken per ATP hydrolyzed? What is the step size? Approaches: single molecule assays, optical traps and high resolution fluorescence analyses Regulation of muscle contraction motor nerve action potential muscle cell plasma membrane depolarized T-tubules (invaginations) carry signal throughout myofibril sarcoplasmic reticulum releases calcium contraction occurs calcium pumped back in over in 30 milliseconds Striated muscle: Calcium regulation of contraction occurs through thin filament accessory proteins: tropomyosin and troponin when calcium binds troponin, tropomyosin shifts to allow actin-myosin interaction Variations: Smooth muscle gut - slower, sustained contractions less ordered myofibrils - no striations less extensive sarcoplasmic reticulum regulation through thin filament dependent on caldesmon regulation also through myosin, still calcium dependent: change in light chain conformation phosphorylation of light chain by MLCK Maintenance of sarcomere structure Why are thick and thin filaments of fixed length? actin capping proteins: Tropomodulin, Cap Z What gives muscle its elasticity? stretch muscle beyond overlap of thick and thin filaments and it resumes resting length when released Giant Muscle Proteins Titin: 3rd most abundant muscle protein M.W. 2,700,000! 25k amino acids. extends from Z-disk to M-line Ig and fibronectin-like domains “super repeats” - myosin binding sites PEVK domains - elastic? Nebulin: M.W. 800,000 helical, wrapped around thin filament repeats that bind actin nebulin length correlates with thin filament length Titin and Nebulin are thought to provide compliance to muscle, and may serve as sarcomere “rulers” determining the length of thick and thin filaments Dystrophin: M.W. 400,000 largest gene - 2 megabases links actin to p. membrane mutation muscular dystrophy