Polymorphism of F-actin assembly. 2. Effects of barbed end capping on F-actin assembly.

In the accompanying paper [Suzuki, A., Yamazaki, M., & Ito, T. (1996) Biochemistry 35, 5238-5244], we presented a quantitative phase diagram of actin filament ( (F-actin) described with the F-actin concentration and delta chi value which characterizes the affinity of F-actin with solvent. The phase diagram shows that F-actin changes its assembly structure from an isotropic disordered distribution to a dilute ordered assembly of a lyotropic crystalline with an increase in the concentration an to a concentrated ordered assembly of a crystalline-like bundle with an increase in the delta chi value (i.e., with a decrease in the affinity with the solvent), respectively, in the physiological concentration range. We report here that capping the barbed end of F-actin significantly affects the phase diagram. The F-actin capped by gelsolin (capped F-actin) decreased the delta chi value required for the formation of the concentrated ordered assembly. The time taken for the decrease in the delta chi value to reach a stationary state after the barbed end capping was proportional to the filament length (approximately 1 h/microm length). the electron microscopic morphology of the concentrated ordered assembly of the capped F-actin was a wide and loose bundle, which was distinctly different from the crystalline-like bundle of the uncapped F-actin. Fragmin from the acellular slime mould, which has similar functions to gelsolin, showed the same effects. These results suggest that the barbed end capping of F-actin gradually changes the nature of the whole filament so as to make the interaction with the solvent more unstable, and the F-actin loses the ability to make a crystalline-like bundle.