muscle_contraction.pdf

Muscle and Contractile Proteins Josh E. Baker, Ph.D. Associate Professor of Biochemistry Howard 163 Office: 784-4103 Cell: 351-4243 [email protected] 1 Outline Actin/Myosin Structure, ATPase, and Mechanics Sarcomere Proteins Muscle Regulation Factors that influence Muscle Force Muscle Diseases 2 Level...

Muscle and Contractile Proteins Josh E. Baker, Ph.D. Associate Professor of Biochemistry Howard 163 Office: 784-4103 Cell: 351-4243 [email protected] 1 Outline Actin/Myosin Structure, ATPase, and Mechanics Sarcomere Proteins Muscle Regulation Factors that influence Muscle Force Muscle Diseases 2 Levels of organization within a skeletal muscle. 3 Myosin Family Tree Actin Based Myosin Motors Tony Hodge and Jamie Cope Myosin Motors Skeletal Muscle Contraction Cell Division (myosin II) Beating Heart (cardiac myosin) Vesicle Transport (myosin V) 5 q There are many myosin types q All have similar motor domains q Differences in tail correspond to differences in cargo and regulation. Diseases Muscle Myopathies Griscelli Syndrome Hearing Loss 6 Non-Muscle Myosins Myosin IIa: Function: Involved in cell division. Myosin V: Function: Involved in melanosome transport and certain neurological functions Disease: Griscelli’s Syndrome. Myosin Va point mutation leads to hypopigmentations and neurological defects. Myosin VI and Myosin VII : Function: involved in maintaining organization of actinfilled stereocilia. Disease: Mutations to myosin VI and VII associated with hearing loss. Certain Myosin VII mutations associated with Usher Syndrome. 7 q Muscle Myosin II is a dimer q Each heavy chain has a motor domain and a tail Actin Binding Site Active Site ELC RLC 8 q All myosin’s are actin-based motors. q All but myosin VI are plus-end directed. 9 Muscle Metabolism Sources for ATP in muscle: - ATP - Creatine Phosphate - Glycogen 14 15 16 ATPase Cycle “The Movie” PG-13 17 Myosin’s working step: A “Lever Arm” Rotation 18 q Myosin V is a high (> 50%) duty ratio “processive” motor q Myosin V transports vesicles in cells 36 nm 19 Myosin V HMM YFP YFP 20 q Myosin II is a low duty ratio (< 10%) motor Myosin V -long neck (6IQ motifs) -organelle motor -functional unit: two heads Myosin II -short neck (2IQ motifs) -drives muscle contraction -functional unit: ~20 heads 21 q Many myosin II molecules required to propel actin filament V = d/Ton Myosin 30 µm In vitro motility assay Real time fluorescence microscopy 22 q In muscle, myosin II molecules are assembled into a thick filament. q Filaments form by association of hydrophobic regions in the tail. 23 q Muscle sarcomere is the fundamental contractile unit in muscle. q The sarcomere contracts when myosin thick filaments and actin thin filaments slide past each other. 24 Accessory proteins in muscle q CapZ and tropomodulin cap ends of actin to keep filament length constant. q Z disc contains a-actinin and other proteins that stably join sarcomeres. q Titin maintains thick filament position in the sarcomere. q Nebulin sets the length of the thin filaments. 25 Figure 16-72 Molecular Biology of the Cell q Tropomyosin wraps around actin filament blocking myosin binding sites on actin q Calcium binding to Troponin C results in tropomyosin movement away from myosin binding sites. 26 27 Effect of Ca2+ on actomyosin ATPase and force 28 Muscle Regulation Two types of regulation: – Thin filament (skeletal and cardiac) – Thick filament (smooth muscle) Smooth Muscle Regulation: Calcium-Calmodulin activates Myosin Light Chain Kinase (inactive) Myosin Myosin-P (active) Myosin Light Chain Phosphatase 29 Factors that affect muscles ability to generate force and to contract. 1. Myosin isoforms 2. Frequency of stimulation 3. Number of motor units stimulated. 4. Degree of stretch (Frank-Starling relationship). 5. Whether muscle is allowed to shorten (Force-velocity relationship). Factors that affect muscles ability to generate force and to contract. 1. Myosin isoforms 2. Frequency of stimulation 3. Number of motor units stimulated. 4. Degree of stretch (Frank-Starling relationship). 5. Whether muscle is allowed to shorten (Force-velocity relationship). Factors that affect muscles ability to generate force and to contract. 1. Myosin isoforms 2. Frequency of stimulation 3. Number of motor units stimulated. 4. Degree of stretch (Frank-Starling relationship). 5. Whether muscle is allowed to shorten (Force-velocity relationship). Asynchronous motor-unit activity maintains a nearly constant tension in the total muscle. Factors that affect muscles ability to generate force and to contract. 1. Myosin isoforms 2. Frequency of stimulation 3. Number of motor units stimulated. 4. Degree of stretch (Frank-Starling relationship). 5. Whether muscle is allowed to shorten (Force-velocity relationship). The “Length-Tension” relationship in a sarcomere (Frank-Starling mechanism) Length-tension relationship in muscle tissue Factors that affect muscles ability to generate force and to contract. 1. Myosin isoforms 2. Frequency of stimulation 3. Number of motor units stimulated. 4. Degree of stretch (Frank-Starling relationship). 5. Whether muscle is allowed to shorten (Force-velocity relationship). Isometric and isotonic muscle contractions Change in the isotonic twitch response of a muscle fiber with different loads. Force-Velocity curves show relationship between shortening velocity and load. Myosins make up 10 – 15% of the protein in the body. Myosin mutations are bound to lead to muscle disorders. 49 Mutations to MyHC IIa (MYHC2): Disease: Muscle Myopathy. Clinical Features: Muscle Weakness. Atrophy near shoulders, back, hand and thigh muscles. Muscle weakness. Pathogenesis: Mutations primarily to SH1 Helix in myosin. Thought to alter actin-myosin ATPase activity. Mutations to embryonic MyHC (MYH3): Disease: Distal arthrogryposis. Freeman-Sheldon Syndrome, Sheldon-Hall Syndrome. Clinical Features: Joint contractures with predominant distal involvement. Pathogenesis: Mutations in troponin I, troponin T, tropomyosin, perinatal myosin and embryonic myosin. Thought to disrupt sarcomere development. 50 Mutations to B-Cardiac myosin (MyHC7): Disease: Laing myopathy. Clinical Features: Weakness of ankle dorsiflexion and “hanging big toe”. Pathogenesis: Mutations are in the LMM region of Myosin. Thought to disrupt myosin filament formation or disrupt interactions with myosin binding proteins like titin. 51 q Point mutations to β-cardiac myosin, actin, troponin, and tropomyosin have been linked to familial hypertrophic cardiomyopathy (FHC) and dilated cardiomyopathy (DCM). 52 Myosin mutations cause “Familial hypertrophic cardiomyopathy” and sudden death Mouse model of hypertrophic cardiomyopathy Wt cardiac myosin R403N mutant myosin Pathogenesis: FHC mutations enhance myosin force generation DCM mutations decrease myosin force generation 53