Harpers

MUSCLE & THE CYTOSKELETON / 559G-actinF-actin6–7 nmTropomyosinTroponin38.5 nmTpCTpITpT35.5 nmThe assembled thin filamentFigure 49–3. Schematic representation of the thin filament, showing the spatial configuration of its threemajor protein components: actin, myosin, and tropomyosin. The upper panel shows individual molecules ofG-actin. The middle panel shows actin monomers assembled into F-actin. Individual molecules of tropomyosin(two strands wound around one another) and of troponin (made up of its three subunits) are also shown. Thelower panel shows the assembled thin filament, consisting of F-actin, tropomyosin, and the three subunits oftroponin (TpC, TpI, and TpT).ure 49–2). Thus, the arrays of interdigitating filamentsmust slide past one another during contraction. Crossbridgesthat link thick and thin filaments at certainstages in the contraction cycle generate and sustain thetension. The tension developed during muscle contractionis proportionate to the filament overlap and to thenumber of cross-bridges. Each cross-bridge head is connectedto the thick filament via a flexible fibrous segmentthat can bend outward from the thick filament.This flexible segment facilitates contact of the headwith the thin filament when necessary but is also sufficientlypliant to be accommodated in the interfilamentspacing.ACTIN & MYOSIN ARE THE MAJORPROTEINS OF MUSCLEThe mass of a muscle is made up of 75% water andmore than 20% protein. The two major proteins areactin and myosin.Monomeric G-actin (43 kDa; G, globular) makesup 25% of muscle protein by weight. At physiologicionic strength and in the presence of Mg 2+ , G-actinpolymerizes noncovalently to form an insoluble doublehelical filament called F-actin (Figure 49–3). TheF-actin fiber is 6–7 nm thick and has a pitch or repeatingstructure every 35.5 nm.