Cilia and flagella are essentially the same things and are highly similar between all eukaryotes. (Bacterial flagella are entirely different and probably shouldn't be called flagella at all.) The inner mechanical works of a flagellum is called the axoneme and is based on microtubules. Although there are exceptions, most axonemes have 9 doublet microtubules organized as a cylinder around 2 central pair microtubules. This so-called 9 + 2 axoneme is diagrammed below.
Control of Flagellar Beating
It is not clear how the activity of dynein motors is controlled. Many lines of evidence indicate that the regulatory mechanisms are quite complex. The central pair microtubules and the radial spoke structures both play important roles in controlling flagellar beating. In addition, there is a complex system of kinases and phosphatases that regulate dynein motors and thus flagellar motility. Evidence also indicates that Ca2+ is an important regulator of beating in most flagella. My lab has been involved in experiments investigating most of these aspects of flagellar control, but our central focus is on the kinase cAMP-dependent protein kinase (PKA).
What We Know About PKA in Chlamydomonas Flagella
In Chlamydomonas, PKA inhibits flagellar beating2, and our early work showed that active PKA slows the dynein motors down3. Subsequent work by our collaborators (Win Sale, Emory University School of Medicine) indicates that one particular form of dynein called "inner arm I1" is the only type of dynein necessary for the PKA effect1. As a kinase enzyme, the normal role of PKA is to phosphorylate proteins. Putting this information together suggests the following model for the role of PKA in regulating flagellar motility.
What We Don't Know About PKA
Although all evidence points to a crucial role for PKA in controlling flagella in Chlamydomonas and other organisms, there is an awful lot that we do not know about flagellar PKA. One other thing we are pretty sure of is that the PKA in Chlamydomonas flagella is a weird form of PKA. Thus, the "rules" about the way PKA works in the cytoplasm of other cells may not apply in Chlamydomonas. Here is a partial list of what we don't know:
What We Are Currently Doing to Find Out About PKA
1) We are in the process of identifying and purifying the PKA protein from Chlamydomonas flagella. We are using an affinity-based approach to this problem.
2) We are in the process of cloning a potential gene for Chlamydomonas PKA. Undergraduates Colleen Trantow and Jason Wells successfully cloned most of the kinase domain of this gene. However, there is some question as to whether this gene is actually a PKA or not. We are in the process of cloning an additional part of the sequence which should greatly help our understanding of this gene.
3) We are conducting experiments to try to determine the role of PKA in controlling flagellar motility. One popular hypothesis is that PKA is required for Chlamydomonas to perform phototaxis.
Former undergraduate researcher Mike Watson tested this hypothesis using lysed-cell models in which the flagella were still attached to the cell body but the membranes were removed. Much to our surprise, he found that changing the activity of PKA or of phosphatases did not change the way that cells respond to Ca2+. His results would suggest that PKA is not involved in phototaxis4. However, there may be other explanations for his results. For example, because these were dead, lysed cells, actual phototaxis could not be tested. Perhaps in an intact cell PKA does play a role. We are currently using other approaches to test the potential role of PKA in living cells. These experiments use a variety of microscope-base motility assays.
This was not intended to be an extensive review of PKA in flagella or flagellar motility. Instead it is a simplistic introduction to what goes on in the Howard lab. Numerous critically important works and concepts were left out for simplicity's sake. My apologies to all the labs that were not cited here.
Copyright © 2005, the University of Wisconsin-La Crosse and the Board of Regents of the University of Wisconsin.
If you have any questions about or additions to these pages please contact firstname.lastname@example.org
Last Modified: July 2005