How to build a bend into an actin bundle.

The bundle of actin filaments in the sperm of the horseshoe crab has a polygonal form. This appearance results from regularly spaced, sharp bends (elbows) separated by straight segments (arms). We conclude that the elbows are built into the bundle by a rearrangement of interfilament crossbridges in the arms. The rearrangement, which makes used of the specific bonding properties of the crossbridges, is made possible by the helical symmetry of actin and by the hexagonal packing of the filaments. The angle of the bend (154 degrees) is fixed by the interfilament separation and the axial spacing of actin subunits within a filament. In the arms on either side of an elbow, rows of filaments slip relative to each other. The slippage, and hence the elbow, is locked in by an axial shift of the crossbridges between adjacent rows of filaments. The variable twist of actin filaments may be important in accommodating the strain of crossbridge rearrangement.