Ciliary and Collective Movement

Figure 5.25: Axoplasmic transport occurs by coordinated activities of sidearm, contractile proteins ("dynein"), which cooperatively pass material in a "bucket brigade." The orchestration mechanism is unknown, but shown here as the consequence of signaling by "soliton" waves of tubulin conformational states. By Fred Anderson.

Figure 5.25: Axoplasmic transport occurs by coordinated activities of sidearm, contractile proteins ("dynein"), which cooperatively pass material in a "bucket brigade." The orchestration mechanism is unknown, but shown here as the consequence of signaling by "soliton" waves of tubulin conformational states. By Fred Anderson.

The structure of cilia and flagella is a core of parallel microtubules in a cylindrical "9+2" arrangement of doublet or triplet microtubules. The arrangement is similar to centrioles which are also cylinders of 9 MT triplets, but without the additional central pair. Motor cilia and flagella have actin wound around their central pair of MT. The side arm proteins which connect the parallel microtubules are called links, spokes, or sidearms and are comprised of dynein, the contractile protein which utilizes ATP energy to produce force. Cilia and flagella are anchored inside the cell to basal bodies which are also composed of parallel microtubules and form a short cylinder with the same outer diameter and nine fold symmetry as cilia, flagella and centrioles. It has been clearly shown that ciliary mechanisms can function without influence from the cell to which they are attached. Flagella or cilia severed from cells by laser beams continue to propagate normal bending movements, if the surrounding medium contains ATP and either magnesium or calcium ions. Cilia function to move fluid over the surface of a cell or to propel single cells through a fluid. Single cell organisms use cilia for the collection of food particles and for locomotion. In the human lung and respiratory tract, epithelial lining contains about a billion cilia per square centimeter which act to sweep layers of mucous, trapped particles, and dead cells towards the mouth where they may be coughed up or swallowed. The cilia bend in coordinated unidirectional waves in which each cilium moves as a tiny whip. There is a forward stroke in which the cilium is extended and exerts maximal force on the surrounding liquid medium, followed by a recovery phase when it returns to its initial position. The movement is a rolling motion which minimizes viscous drag and requires about 0.1 to 0.2 seconds. Cycles of adjacent cilia are not quite in synchrony, and the small delay produces wave-like patterns of the entire ciliary complex. Simple flagella of sperm and single cell organisms are much like large cilia in their internal structure but are usually longer and propagate in quasi-sinusoidal waves rather than faster whip-like movements. The mechanism of flagellar beating in eukaryotes (but not of bacteria) is very similar to that of cilia.

Muscle contraction, axoplasmic transport, and ciliary and flagella motion all occur by the contractile bending of sidearm proteins attached along cytoskeletal filaments. In the case of muscle contraction, the stable cytoskeletal filaments are myosin with appendages called myosin heads crawling along parallel actin filaments. In the case of axoplasmic transport and ciliary and flagellar bending, microtubules are the stable filaments from which dynein contractile sidearms crawl or row along other MT (in the case of cilia and flagella), or pass along material or vesicles (in the case of axoplasmic transport). The energy for all of these mechanisms is supplied by the hydrolysis of ATP, but its precise utilization, transfer of conformational states and the temporal orchestration required to control these sidearm appendages are unknown. Figure 5.25 shows one possible cooperative control mechanism in which a wave of tubulin conformational states (soliton) travels along the MT cylindrical surface lattice to trigger the dynein activities.

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