Diagram of a sarcomere bounded by the Z-bands. The left side (peach color) of the sarcomere represents a half sarcomere found in vertebrate skeletal myofibrils. Note that the nebulin molecules are part of and extend the entrie length of the thin filaments. The right side (pink color) of the sarcomere reflects a half sarcomere in cardiac muscle cells. The smaller nebulin isoform, nebulette, begins within the Z-band and extends only a short distance along the thin filament. Titin is shown with its NH2 termini from adjacent sarcomeres overlapping in the Z-band. Groups of three titin filaments are shown aligned together in the half sarcomere and overlapping in the Z-band. Groups of three titin filaments are shown alighned together in the half sarcomere and overlapping in the M-band with groups of three from the other half sarcomere. The scale of the drawing does not allow the ratio of six titins per half-thick filament or the two nebulin isoforms per actin thin filament to be illustrated. The double-headed arrows indicate the position of the region of titin used as a bait to pull out obscurin. The M-band is the mid-point of the group of aligned thick myosin filaments (A-band) where obscurin binds in cultured neonatal cardiomyocytes and in adult muscles.
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