Exam 3 Review:  Chapter 09:  Sarcomere Composition

elastic filament = titin = connectin - An enormous elastic protein (2500-3500 KD) found in the sarcomere of striated muscle cells; these proteins form a scaffolding important for the correct assembly of the sarcomere; each titin molecule spans the distance from M line to Z disc and contributes to the passive tension in muscle cells as well as keeping the thick myosin filaments centered.

thin filaments = actin fibers - The 7-9 nm diameter contractile proteins attached to the Z discs of striated muscle sarcomeres, arranged with opposite polarity in each half sarcomere; composed of the protein subunits F-actin, troponin, and tropomyosin; these contractile proteins are found in the cytoskeleton of smooth muscle and some other tissues as well.

actin - A contractile protein which is the main constituent of the thin filaments; it is involved in muscle contraction and an important cytoskeletal element in many other cell types; consists of globular = G-actin subunits which combine to form F = filamentous actin strands which then will further combine with troponin and tropmyosin proteins, to form the fully functional thin filament; thin filaments are anchored together at the Z line = Z disc of the sarcomere.

troponin - A protein monomer associated with the thin actin filaments involved in striated muscle contraction; this protein complex consists of three functional subunits, one portion which has receptors for calcium ions and two portions which bind the protein to the other two protein components, actin and tropomyosin,  of the thin filament; there is one troponin associated with each tropomyosin protein.  Nickname:  the "switch molecule."

tropomyosin - A fibrous protein monomer associated with actin filaments in the sarcomere of striated muscle cells and in the cytoskeleton of most cells; composed of two helical chains that span the length of 6 or 7 G-actin molecules; there is one tropomyosin associated with each troponin protein; tropomyosin helps stabilize the actin fiber and participates in the binding of actin to myosin.

myosin-binding site - The specific location on each G-actin molecule where a myosin head can covalently bond to the actin filament during muscle contraction; at rest the myosin-binding site is covered by the tropomyosin molecule, but during contraction, the presence of calcium ions causes a conformational change in the tropomysoin molecule so that it shifts position and exposes the myosin-binding site.

thick filaments = myosin fibers - The larger diameter contractile proteins anchored to the M lines of striated muscle sarcomeres, arranged with opposite polarity in each half sarcomere; composed of the protein subunit myosin; these contractile proteins are found in the cytoskeleton of smooth muscle and some other tissues as well.

myosin - The contractile protein monomer comprising myosin fibers which consists of a pair of intertwined strands forming the "tail" and the active "head" of each strand which is the specific site for both attachment of myosin to actin and for the repeated power strokes, due to its ATPase activity, which achieve the sliding of actin fibers over the myosin fibers during muscle contraction; other related myosin-family proteins are associated with the cytoskeleton and with internal cytoplasmic transport processes.

myosin heads - The pair of active subregions of each myosin protein monomer which have the capacity to bind and to hydrolyze ATP, to form covalent cross-links with actin filaments, and to perform the power strokes which produce the sliding of filaments that achieves contraction of the sarcomere.

myosin tails - The pair of subregions of each myosin protein monomer which align with other myosin monomer's tail regions to produce the huge polymer which is the myosin fiber or thick filament.

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