Practice question

Actin filaments crosslinked into distinct architectures produce diverse mechanical properties. Short crosslinkers like fimbrin fascin 8 to 12 nanometer spacing generate tight parallel bundles in microvilli and filopodia for protrusion, long crosslinkers alpha actinin filamin 30 to 40 nanometer create loose networks and antiparallel bundles allowing myosin II insertion for contraction. Alpha actinin antiparallel homodimer 35 nanometer rod contains N terminal actin binding domain with paired calponin homology CH1 CH2, four spectrin repeats dimerization core and C terminal calmodulin like EF hands regulated by PIP2 calcium. Each dimer binds two filaments at distance matching spectrin repeat length. In Z disc alpha actinin crosslinks antiparallel thin filaments, in stress fibers alternating polarity bundles, in focal adhesions interacts with vinculin zyxin linking to integrins. Mechanical unfolding of spectrin repeats under tension provides elasticity. Tropomyosin binds along groove not crosslinking, profilin monomer regulation, Rho GTP upstream signal not structural. Hence alpha actinin recognized as major physiological crosslinker establishing both meshworks and contractile bundles essential for force transmission.