Alpha-actinin increases actin filament end concentration by inhibiting annealing.

The usual rate of actin polymerization is increased if one starts from actin nuclei. We have noticed that, using alpha-actinin crosslinked actin nuclei, the initial net elongation rate is further enhanced. Also initial net depolymerization rates of alpha-actinin crosslinked F-actin samples are higher than those of controls. These results should imply that alpha-actinin increases the filament end concentration of actin samples. The experiments with barbed and blocking substances (cytochalasin D and gelsolin-actin complex) confirmed such an increase. We have shown that: (1) alpha-actinin does not significantly influence actin polymerization over all; (2) alpha-actinin inhibits the recovery of the filament size in F-actin samples after sonication; and (3) the influence of alpha-actinin on actin filament end concentration is counteracted by tropomyosin. Therefore, we suggest that, upon filament shearing, alpha-actinin crosslinking inhibits the annealing of short actin polymers into longer filaments.