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From spermatozoa motility to the passage of mucus through the airways, the movement of cilia and flagella is vital for numerous physiological functions and has long fascinated biologists (see Milestone 22). Efforts to understand this motility led to some of the fundamental discoveries in cell biology in the twentieth century.
In the 1950s, the blossoming field of electron microscopy had allowed the structure of the axoneme — the structural core of cilia and flagella — to be visualized, revealing the now familiar arrangement of nine microtubule doublets, linked by protein 'arms', surrounding a central microtubule pair. However, little was known about the protein components of the axoneme.
Axoneme-isolation studies had demonstrated that axoneme motility required an ATPase. In 1965, Gibbons and Rowe identified an ATPase with enzymatic and structural properties matching those of the axoneme arms within the ciliated protozoon Tetrahymena pyriformis. They named the protein dynein (from the Greek dyne, meaning force). This marked the discovery of the first microtubule motor protein and 20 years would pass before another — kinesin — would be identified (see Milestone 15). Three years after the identification of dynein, another component of the axoneme was identified by Mohri, who isolated and characterized the protein subunit of sea urchin spermatozoa microtubules and named it tubulin.
... when Gibbons presented this work at the annual ASCB meeting, the movies of the ATP-induced sliding of axonemal microtubules were given a standing ovation by the audience
Gregg Gundersen
In subsequent years, attention turned to the mechanism of cilia bending. The 'sliding filament model', which proposed that microtubules actively slide along each other rather than individually contracting, was gaining support, largely due to the demonstration by Satir that microtubule length remains constant during bending. In 1971, Summers and Gibbons provided a notable visual demonstration of the model in action.
Summers and Gibbons were studying the sea urchin spermatozoa axoneme. Trypsinization sensitized the isolated axoneme so that the addition of ATP caused it to disintegrate rapidly. Using dark-field microscopy, Summers and Gibbons recorded the disintegration, revealing sliding movements between microtubule doublets, and the protrusion and expulsion of individual doublets from the axoneme. These experiments helped to ensure the firm acceptance of the sliding filament model.
Today, it is known that microtubule-mediated transport is crucial for many aspects of cellular function. In addition to the large family of dyneins that drive axonemal motility, cytoplasmic dynein, which was discovered in 1987, has multiple roles throughout the cell, from the control of mitosis to the transport of cargo.
