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2. Bio2-Cytoskelette-2-Myosin Proteínes Moteur : la Myosine Structure des divers Types de La proteolyse de la Myosine II révèle Myosine les differents domaines de structure Figure 18-20. Structure of various myosin molecules. (a) The three major myosin proteins are organized into head, neck, and tail domains, which carry out different functions. The head domain binds actin and has ATPase activity. The light chains, bound to the neck domain, regulate the head domain. The tail domain dictates the specific role of each myosin in the cell. Note that myosin II, the form that functions in muscle contraction, is a dimer with a long rigid coiled-coil tail. (b) (b) Proteolysis of myosin II reveals its domain structure. For example, most proteases cleave myosin II at the base of the neck domain to generate a paired-head and neck fragment, called heavy meromyosin (HMM), and a rodlike tail fragment, called light meromyosin (LMM). Further digestion of HMM with papain splits off the neck region (S2 fragment) and leads to separation of the two head domains into single myosin head fragments (S1 fragments). (Fuente: Lodish et al., 2000) Microvillosités MYOSINES Figure 16-77. Freeze-etch electron micrograph of an intestinal epithelial MICROViLLOSITES cell, showing the terminal web beneath the apical plasma membrane. Bundles of actin filaments forming the core of microvilli extend into the terminal INTESTINALES web, where they are linked together by a complex set of cytoskeletal proteins that includes spectrin and myosin-II. Beneath the terminal web is a layer of intermediate filaments. (From N. Hirokawa and J.E. Heuser, J. Cell Biol. 91:399-409, 1981) (Fuente: Alberts et al., 1993) 1 2 • Actin • Myosin • Actinin (z-disc) Figure 18-21. Functions of the Organisation des Fibres d’Actine et de myosin tail domain. (a) Myosin I Myosine dans le Muscle strié et lisse and myosin V are localized to Fonctions des domaines de la queue de la Myosine cellular membranes by undetermined sites in their tail domains. As a result, these myosins are associated with intracellular membrane vesicles or the cytoplasmic face of the plasma membrane. (b) In contrast, the coiled-coil tail domains of myosin II molecules pack side by side, forming a thick filament from which the heads project. In a skeletal muscle, the thick filament is bipolar. Heads are at the ends of the thick filament and are separated by a bare zone, which consists of the side-by-side tails. (Fuente: Lodish et al., 2000) La structure du sarcomère musculaire Figure 18-26. General structure of skeletal and smooth muscle. (a) Skeletal muscle tissue is composed of bundles of multinucleated muscle cells, or myofibers. Each muscle cell is packed with bundles of actin and myosin filaments, organized into myofibrils that extend the length of the cell. Packed end to end in a myofibril is a chain of sarcomeres, the functional units of contraction. The internal organization of the filaments gives skeletal muscle cells a striated appearance. (b) Smooth muscle is composed of loosely organized spindle-shaped cells that contain a single nucleus. Loose bundles of actin and myosin filaments pack the cytoplasm of smooth muscle cells. These bundles Figure 16-89. The myosin and actin filaments of a sarcomere overlap with the are connected to dense bodies in the cytosol and to the membrane at attachment same relative polarity on either side of the midline. (Fuente: Alberts et al., 1993) plaques. (Fuente: Lodish et al., 2000) 3 Interactions competitives et cooperatives entre les proteínes de réticulation et l’Actine Figure 16-78. Some examples of competitive and cooperative interactions between actin-binding proteins. The arrowhead at the end of each actin filament indicates the minus end. Tropomyosin and filamin both bind strongly to actin filaments, but their binding is competitive. Because tropomyosin binds cooperatively to actin filaments, either tropomyosin or filamin will predominate over large regions of the actin filament network. Other actin-binding proteins, such as α-actinin or myosin-II, will be excluded from specific sites by a competitive interaction; thus, for example, α-actinin binds all along pure actin filaments in vitro, but it binds relatively weakly to actin filaments in cells, where it is largely confined to sites near the plus ends because of competition with other proteins. Alternatively, binding can be enhanced through cooperative interaction; thus tropomyosin appears to enhance the binding of myosin- II to actin filaments. Multiple interactions of these types between the many different types of actin-binding proteins are thought to be responsible for the complex variety of actin networks found in all eucaryotic cells 4 -Troponin I- TROPONIN C Functions: * One of three subunits that form the troponin complex in striated muscle * Binds to actin to aid in muscle contraction Clinical Usage: * Elevation is a sign of end-stage renal disease * Serum biomarker for acute myocardial infarction Mutations of troponin: * Leads to Familial Hypertrophic Cardiomyopathy * Wolf-Parkinson-White Syndrome (mutation of chromosome 3) Troponin C TROPONIN C 5 Tropomyosine TROPONIN C Cycle de la Myosine Figure 16-91. The cycle of changes by which a myosin molecule walks along an actin filament. (Based on I. Rayment et al., Science261:50-58, 1993. © 1993 the AAAS.) 6 7 PROTEINES de la BANDE Z α-Actinin - NspI TITINE TITINE 8 NEBULINE NEBULINE NEBULINE • biggest actin binding protein (600-900kDa) • binds a potential 200 actin monomers! • Each monomer is bound by 35 AA (may also bind calmodulin, tropomyosin and troponin) - the N-terminal binds tropomodulin •actin-binding core : conserved motif SDxxYK • interaction with actin: is Ca2+-calmodulin sensitive • forms a "molecular ruler" controlling the length of the thin filament • associated with an autosomal recessive nemaline myopathy in humans 9 DYNAMINE Fimbrin+Urotrophine/Calponine = GTPases : role in endocytosis & trafficking DYNAMINE Vesicle Trafficking Myosins • Myosin I – participate in growth cone formation, not associated with vesicles in the CNS. • Myosins VI and VII – play integral role in sensory cells (retina and stereocilia), no apparent role in central neurons and glia. • Myosin V – abundant in CNS and vesicle associated 10 Key features of the myosin 5a dimer MODEL OF LIGAND-DEPENDENT ]VESICLE DOCKING TO MYOSIN 5 syndapin Plasma dynamin membrane Vesicle docking Arp2/3 endosome TRΔα2 1°endosome cofilin Tail PEST WASP Myosin 5a Coiled coil endosome Neck IQ endosome in transit Head Motor Actine monomère ACTIN MOVEMENT 11 12 13.
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