Relationship between the flexibility and motile activity of actin filaments: effect of pH

Arii, Y., Hatori, K.

Department of Bio-System Engineering, Yamagata University


We examined both the flexibility and the sliding movement of actin filaments under a fluorescence microscope because the dynamic property of the filamental structure seems to be significant for transforming of forces exerted by myosin molecules into coordinating unidirectional movement of the actin filaments. In this study the performance of actin filaments was tested at various pH conditions in polymerization and/or in observation since ambient pH influences the filamental stability.
After Ca-bound actins were polymerized into the filaments in the pH range from 5.0 to 9.0, these were labeled with rhodamine-phalloidins. Then the actin filaments were applied to a motility assay system. The velocity of filaments was gradually increased with an increase of pH both in polymerization and in observation, and reached to its maximum at pH 8.0 although the velocity was independent of pH only in polymerization.
The contour length and the end-to-end distance of filaments were directly measured in fluorescent images. The persistence length of filaments (as a parameter associated with flexural rigidity) was estimated by referring to the relation between the contour length and the end-to-end distance. The persistence length was increased from 4 um to 9 um with an increase of pH in both conditions over the range of 5.0 - 9.0.
The build-up of rigidity of an entire filament which is postulated as an elastic rod, was found to be accompanied by enhancement of the sliding movement as for Ca-F-actin.