Wavelike motions of cytoskeletal fibrils and their mechanics.

Abstract

Actin filaments and microtubules can slide and translocate particles along as it is well known. Moreover, bendings, corners, regions of branching and crossbridges can move in a wavelike manner along bundles of cytoskeletal elements. This has been demonstrated by microcinematography e.g. of ringlike closed F-actin bundles ("waving polygons") in cytoplasmic drops squeezed out of characean internodial cells (Jarosch 1960) and by microtubule bundles of axostyles of Pyrsonympha (Langford and Inoué 1979), or by the rootlet fibril (costa) of Trichomonas (Amos et al. 1979). Single isolated microtubules from squid giant axons that become visible by video-enhanced interference contrast microscopy can glide on glass slides and start a kind of "fishtailing" when gliding is prevented by an obstacle (Allen et al. 1985). The described wavelike motions cannot be explained by the power-stroke or rowing-stroke model of myosin-, kinesin-, or dynein-crossbridges between filaments or microtubules--thus the problem of proper coordination and localization of the single power-strokes is unsolved. The motions can be explained and simulated in detail by macroscopic models with rotating steel helices. This indicates the existence of quickly rotating cytoskeletal elements. Two types of mechanisms are possible: 1) The propagation of angles and corners may depend on the close contact between the rotating elements of the bundle, e.g., by mutual winding and unwinding of actin-associated filaments or microtubule-associated filaments (characean polygons, axostyles of Pyrsonympha). 2) The rotating elements of the bundle form superhelices, and their rotation results in microscopic helical waves (bacterial flagella, helical filopodia, "corkscrewing" of a helical bundle). Eucaryotic flagella transform the latent helical waves of their helically shaped doublet microtubules and the central singlet helix to large helical or uniplanar bending waves by a most intricate mechanical coil-coil interaction that is demonstrated in a simplified manner by model experiments.