Practice question

Organization of sarcomere depends on anchoring thin filaments at Z disc providing mechanical integrity during repetitive contraction. Z disc 100 nanometer thick electron dense structure contains alpha actinin antiparallel dimer crosslinking antiparallel barbed ends from adjacent sarcomeres into orthogonal lattice spaced 20 nanometer, plus titin Z repeats, telethonin, myotilin, FATZ. Nebulin huge filamentous protein 600 to 900 kilodalton with 185 repeats of 35 residue nebulin motif SDXXYK each binding one actin monomer, plus N terminus interacting with tropomodulin capping pointed end and C terminal SH3 domain binding myopalladin CapZ inside Z disc embedding. By acting as molecular ruler nebulin dictates thin filament length and stabilizes filament against cofilin severing and depolymerization, loss causing nemaline myopathy with shorter thin filaments. Spectrin provides membrane skeleton in erythrocytes, dystrophin links actin via cysteine rich domain to dystroglycan complex at costameres for lateral force transmission, not Z disc anchoring. Titin provides elasticity and scaffold but nebulin specialized for actin Z disc anchorage and length specification.