Wednesday, September 13, 2006

Axoneme Structure

A linked article entitled 'Secrets of a Cellular Machine New Clues to the Architecture of Flagella and Cilia' provides information about axoneme structure. The article provoked more than the usual interest because of a recent paper co-authored by Nick Matzke. It was the subject of a blog post at Telic Thoughts. Portions of 'Secrets of a Cellular Machine New Clues to the Architecture of Flagella and Cilia' follow in italics accompanied by my comments in block print.

Kenneth Downing and Haixin Sui of Berkeley Lab's Life Sciences Division have pioneered the use of cryo-electron tomography to examine the ubiquitous protein structures called axonemes, which form the cores of the cilia and flagella of eukaryotic cells.

A new model of axoneme structure reveals the roles of specific proteins in organizing microtubule doublets. The model, above, incorporates known tubulin structures docked within the constituent microtubules. Cryo-electron microscope images of doublets in axonemes, like those used for tomography, are shown at bottom.

Axonemes are some of nature's largest molecular machines. Their principal structural elements are microtubules, tough and versatile protein assemblies that perform many cellular roles, notably as major components of the cell skeleton. In 1998 Downing and Eva Nogales, then a scientist in his group, with colleague Sharon Wolf, first revealed the structure of alpha and beta tubulins, the protein dimers from which microtubules are constructed. In 2002 Downing and Huilin Li, also a scientist in his group, published details of a microtubule's structure at eight-angstrom resolution, better than twice that ever obtained before.

"In the present work Haixin Sui and I were initially looking to follow up the earlier work on tubulin," Downing says. "In mammals tubulin comes in many forms, so we intended to isolate the simple form in sea urchin eggs in hopes of making better crystals. It turned out that we also collected a lot of sea urchin sperm, which are an excellent source of axonemes."

Whips and eyelashes
Lacking legs or flippers, many single-celled eukaryotes (eukaryotic cells are those with nuclei) get around using flagella and cilia, Latin for "small whips" and "eyelashes." Nor could complex creatures, including human beings, survive without these powerful molecular machines. The cilia that sprout thickly from cells that line the lungs and other organs wave as rhythmically as sea grass in the tide to dislodge and sweep away litter. Flagella thrash energetically to propel sperm.

Except for length and number per cell, flagella and cilia are similar and share a common structure. At the center of each is the axoneme, a tough, flexible bundle of microtubules encased by a membrane. Other proteins connect the microtubules in the axoneme together or move over them, causing them to bend and slide against each other in a rhythmic beating motion.

"The basic axoneme plan has been known for forty years, from biochemistry and low-resolution electron microscopy," says Sui. "But finding out which proteins are located where, and even learning the identities of many of the proteins, has long frustrated researchers."

"Resolution was the challenge," Downing says. "Conventional electron microscopy just couldn't see the details." The latest high-resolution results with cryo-electron tomography offer new insights and promise new understanding of these vital cellular structures.

Axonemes are the giant molecular machines that make up cilia and flagella. The axonemes used in the present study were taken from the sperm cells of purple sea urchins.

Despite the battle lines drawn over Behe's irreducibly complex cilia and flagella the article reminds us that there is much unknown data relating to the issue. Interestingly, although science involves the testing of hypotheses and the incorporation of newly gained knowledge resulting from such testing, advocates for and against Behe's views behave as if any new data could not matter.

Says Downing, "The axoneme is a basic structure in all eukaryotes. Our eventual goal is to find out how it evolved and why it has been conserved since the beginning."

I'd like to see this approached with a truly open mind that is has not predisposed to the idea that there are always plausible pathways without intermediates lacking selective value.


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