The cilium as a biological nanomachine

I first met Keith Porter in 1956 when he was a young man of 44. In the summer of 1956, he was invited to give a Friday evening lecture at the Marine Biological Laboratory at Woods Hole. Although the lecture was called “The Submicroscopic Morphology of Protoplasm,” Porter took the cilium as one of his main examples. I was captured by this talk (1) and reaffirmed my decision to work as a graduate student with him at the Rockefeller Institute for Medical Research in New York to try to understand how the structure of the cilium was related to its mechanism of movement. Studying a variety of cilia, from protozoan to mammalian somatic cilia, D. W. Fawcett, Albert Sedar, and Porter (2, 3) established at least two critical points: The first was that the ciliary filaments (microtubules, as we now call them) were continuous extensions of the basal body filaments surrounded by an extension of the cell membrane, the ciliary membrane. The inside of the cilium was therefore a specialized part of the cell cytoplasm. The second point was that the 9 1 2 pattern of microtubules found in motile plant cilia (4) was arranged consistently and distinctively in all motile cilia as nine doublet microtubules surrounding a central pair. The 9 1 2 pattern of the axoneme surrounded by the ciliary membrane was seen in cross-sections of both Paramecium and vertebrate cilia. With the passage of forty years, we have learned something about the relation of structure to how cilia move. A contemporary standard thin-section (Fig. 1) shows the familiar outer and inner arms, spokes, and so on that characterize the axonemal substructure. Axonemes from mussel gill have a special conspicuous diamond-shaped bridge in one position that defines doublets 5 and 6. This permits us to number each doublet 1–9 in the axoneme in an unequivocal manner. This sort of knowledge was critical in the development of the sliding microtubule model of ciliary motility, which states that motion of the cilium is produced when the arms cause doublet microtubule N 1 1 to slide tipwards with respect to doublet N (5). Gibbons et al. have shown that the arms are members of the dynein ATPase superfamily of (2) end directed molecular motors (6). Given the mechanical constraints imposed by linkages within the axoneme such as the radial spokes, this sliding is translated into a series of bends that then propagate along the length of the organelles to give the characteristic forms of movement. To clarify the details of how sliding is converted into beating, Michael Holwill and I have embarked on a project to model the axoneme accurately by computer modeling techniques at a resolution approaching 4 nm (7, 8). In some ways this is the obverse of the approach of Mary Porter et al. (e.g., ref 9) who have used subtractive image analysis to study structural defects associated with specific axonemal mutations. Our idea has been to use a variety of electron microscopic images to construct a 3-dimensional axoneme, defining each structure as specifically as possible, and then to empower the model by providing the appropriate dynein step to see which structures must change and to predict how they must change during bending (10). To do this at high resolution we are now using an engineering approach, finite element theory. In our current computer image of the axoneme (8), all the familiar structures are present, and the detailed match to electron micrographs taken with a variety of procedures is excellent. Such modeling is necessary and important, because otherwise it has proven impossible to comprehend what happens to individual structural elements of the axoneme when some localized dynein arms produce force. To illustrate the complexity, Fig. 2 shows a view of our current computer model. Any physical prediction can be reflected in the model and tested by comparison to, for example, HVEM of quick-fixed bent cilia. This image shows only a small portion of the typical 10 mm long axoneme. There are, for example, ;400 outer dynein arms (odas) along a single microtubule and a more or less similar number of inner dynein arms (idas). Although the odas are thought all to be identical in isoform composition and equivalent in structure, there are several different types of idas per spoke group repeat. The pattern of activity of this ensemble of dynein arms is crucial for an understanding of ciliary motion, and, as a first step in empowering