Motions of Eukaryotic Cilia and Flagella

Cilia and flagella are structures on the outer surface of eukaryotic cells that are involved in moving cells. Cilia are shorter than flagella and exert a coordinated rowing motion to move a microorganism through a solution. Flagella are longer and propel the cell by an undulatory motion.

Both cilia and flagella contain a highly organized bundle of microtubules called an axoneme, enveloped by an extension of the plasma membrane and connected to a basal body, an anchoring structure within the cell (Figure 8.22b).

Axonemes have microtubules arranged in a so-called 9 + 2 array-two central microtubules ringed by nine microtubule doublets (Figure 8.22b). The single microtubules in the center are complete, each having 13 protofilaments of tubulin dimers (Figure 8.19). By contrast, each of the nine surrounding doublets is composed of one complete microtubule (the A fiber) to which is fused an incomplete microtubule, carrying only 10 or 11 protofilaments (the B fiber). Closer inspection of electron micrographs reveals even greater complexity, as diagrammed in Figure 8.23. The outer doublets are periodically interconnected by a protein called nexin and carry at regular intervals sidearms composed of the protein dynein. In addition, radial spokes, each consisting of a head and an arm, project from the outer doublets to connect with the central pair.

About 200 polypeptides can be resolved by gel electrophoresis studies of isolated axonemes. Analysis indicates at least 6 proteins in the spoke heads and 11 others in the arms of the spokes. Much of this apparatus seems to be directly involved in the beating motions of cilia and flagella.

If ATP is added to isolated axonemes, adjacent doublets can be seen to slide past one another. The best current model holds that this sliding occurs by "walking" of the dynein sidearms along the adjacent doublet (Figure 8.24). Doublets slide past each other first on one side of the axoneme and then the other, with the length of the slide limited by the central spokes and nexin connectors. In this way, the sliding of doublets is transformed into back-and-forth bending of the whole cilium or flagellum (Figure 8.24). If connections within the axoneme are removed by careful proteolysis, ATP simply causes axonemes to extend and thin, as the outer doublets slide past one another with no stopping point.

It has been demonstrated that dynein has ATPase activity, with binding of ATP associated with the breaking of dynein cross-bridges. Thus, there are similarities between the mechanisms of the beating of cilia and flagella and the ATP-driven walking of myosin heads along the actin fiber, but there appears to be no relationship between the two systems at the level of protein structure.

See also: Microtubule Systems, Intracellular Transport of Materials