The effect of filament shortening on the mechanical properties of gel-filtered actin.

To address the claim that filaments polymerized from highly purified (gel-filtered) F-actin acquire the elastic properties of a solid attributable to chemical cross-linking, we measured the rheologic spectrum of the dynamic storage modulus, G', and loss modulus, G'' from 5 x 10(-4) to 0.5 Hz for gel-filtered actin alone and in the presence of the actin shortening protein, gelsolin. We confirmed that gel-filtered filamentous actin is a highly elastic material as evidenced by a relatively frequency-independent G', which is consistent with either topologically constrained filaments or a chemically cross-linked gel. Introduction of gel-filtered actin oligomers, however, caused the behavior of gel-filtered actin to become more frequency-dependent and almost identical to that of non-gel-filtered actin, suggesting that the effect of gel filtration on the mechanical behavior of actin is topologic. This conclusion is further supported by the finding that shortening of the actin filaments by the addition of gelsolin at molar ratios to actin of from 1:8000 to 1:500 causes a gradual decrease in elasticity and increase in the amount of flow.