Chlamydomonas

By NM

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You’ve made it to the final round on your favorite TV game show, and you nervously await for the million-dollar question. The lights are blinding, your feet can’t keep still, and all that you’re hoping for is that your degree in biology will finally be useful for something. The studio audience has been prompted to be be silent as the host reads aloud the final question..

“This organelle is found in both prokaryotic and eukaryotic microorganisms, and confers motility by undulat--”

“FLAGELLA!” you exclaim.

FAST FORWARD. The excitement of winning the grand prize has subsided, and more importantly all of your student debts have been paid. But admit it, the question was easy. We all know the role that flagella play in motility. But what else could we say about these organelles in terms of function? Are you at a loss? Contrary to popular knowledge, flagella are actually dynamic structures that exhibit many fascinating characteristics. In eukaryotes, the green algae Chlamydomonas spp. have become a model for studying flagellar processes which include roles in mating, signal transduction, protein trafficking, and sensory reception.  So put on your learning hat, and hold on tight because without further adieu, we're going to explore a few of these functions in greater detail.

Mating in Chlamydomonas is somewhat of a mess but it might appeal to those who think that courtship practices are tacky, or overly formulaic. Instead, Chlamydomonas rely on random collisions between two swimming cells, but not just any encounter will suffice. No, both cells must first be primed for mating by a process called gametogenesis, after which colliding cells may adhere to one another. This adhesion requires that one participant is of 'mating-type plus' and that the other is of 'mating-type minus.' Interestingly, this differentiation of mating-types is determined by flagellar glycoproteins, called sexual agglutinins. The interaction between sexual agglutinins of opposite types does not just cause adherence between the two cells, but also initiates a signal cascade leading to events that recruit additional proteins from the cell body to the flagella, and after which the mating cells ultimately fuse [1].

The transport of proteins and molecules through the flagella in Chlamydomonas is not just limited to mating however. In fact this transport is an interesting and essential consequence of the organism's inability to synthesize proteins within the flagella interior. During flagellar assembly then, Chlamydomonas must synthesize the structural building blocks needed in the cell body, and transport them to the distal end of the flagella where they are added to the growing structure. The trafficking occurs along microtubules, and involves the activity of two proteins--a kinesin which transports molecules into the flagella, and a dynein which returns molecules to the cell for recycling [1]. This process, known as intraflagellar transport (IFT), was first identified in Chlamydomonas but has more recently been found to occur in a range of organisms, including the ciliated cells of mammals.  An elegant method has been developed by researchers to identify molecules being shuttled by IFT, which allows them to shut down either incoming or outgoing transport in real time.  A few interesting proteins that have been found undergoing flagellar trafficking in Chlamydomonas include TRP11 and phototropin.

Chlamydomonas

TRP 11 belongs to a larger family of receptor proteins that can be found across the surface of Chlamydomonas.  Researchers are able to pinpoint the precise location of a protein by creating florescently labeled antibodies that bind specifically to the protein of interest.  While many of the TRP proteins are observed throughout the cell body, immunofluorescent staining has found that TRP11 is localized exclusively on the flagellar surface. The function of TRP11 relates to an interesting behavior of Chlamydomonas--when cells collide with objects in their environment, they typically exhibit a rapid reversal in swimming direction. When expression of the TRP11 protein is artificially inhibited under laboratory conditions, the reflexive behavior is completely abrogated [2]. By this account then, the flagella not only provides the cell with propulsion needed for motility, but also participates in mechanosensory to avoid obstacles within the environment.

Being photosynthetic organisms, it's no surprise that many of the behaviors and cellular processes in Chlamydomonas species are regulated according to the availability of sunlight, which is detected by specialized membrane proteins on the cell exterior.  One such protein is the blue-light receptor phototropin which, surprisingly, has been found within the Chlamydomonas flagella.  And while the phototropin receptor can be found distributed throughout the organism's body, it is typically associated with the plasma membrane. In the flagella however, fractionalization experiments have found that phototropin does not associate with the membrane, but instead separates with the microtubule cytoskeleton. Furthermore, the concentration of phototropin is found to be eight times higher in the flagella than the concentration observed in the cell bodies [3]. Together these findings suggest a novel function for the light receptor that differs from it's usual role in cell bodies.  Although no work has yet established phototropin activity in the flagella, it's presence along with the activity of TRP11 implies that the flagella is an important sensory structure utilized by Chlamydomonas to navigate environments, and coordinate cellular activities.

So there you have it--by using processes observed in Chlamydomonas, we've shown that flagella are important for mating behaviors, initiating signal cascades, trafficking organic molecules, mechanosensory, and potentially photosensory.  And go ahead and consider yourself an expert (sort-of); brag to your friends (no, don't do this); and bask in your new-found appreciation for this microscopic appendage (alright fine, but don't get too carried away).  But most importantly, if you ever find yourself in the hot-seat of your favorite TV game show and they ask perhaps a more difficult question, like: "What organelle presents the sexual agglutinins required for successful adhesion after gametogenesis during Chlamydomonas mating?" or "What structure contains the mechanosensitive TRP11 protein imported by kinesin-mediated IFT?" You can calmly answer..

Flagella.

References

1. Pan J, Snell WJ. Kinesin-II is required for flagellar sensory transduction during fertilization in chlamydomonas. Molecular Biology of the Cell 2002 April 01;13(4):1417-26.

2. Fujiu K, Nakayama Y, Iida H, Sokabe M, Yoshimura K. Mechanoreception in motile flagella of chlamydomonas. Nat Cell Biol 2011 05;13(5):630-2.

3. Huang K, Kunkel T, Beck CF. Localization of the blue-light receptor phototropin to the flagella of the green alga chlamydomonas reinhardtii. Molecular Biology of the Cell 2004 August 01;15(8):3605-14.