ReviewRegulation of flagellar length in Chlamydomonas
Introduction
Cells are able to carefully measure and regulate not only their own size but also that of their organelles. This regulation of size and thus volume is essential for cellular viability and reflects the changing environment of the cell. For example, the size and number of peroxisomes varies depending on the nutrients available for metabolism [1]. Regulation of organelle size is perhaps most apparent in the regulation of flagellar (ciliary) length. Cilia and/or flagella are microtubule-based organelles that project from the surface of cells. Cilia and flagella are identical in structure and in this review are considered to be interchangeable terms. The exquisite control that cells have over flagellar length is perhaps best demonstrated by the unicellular eukaryote Chlamydomonas reinhardtii. Chlamydomonas maintains its two apically localized flagella at an equal length. In addition, within a population of cells there is little variation in flagellar length.
In contrast to mammalian systems, flagella are not essential for viability in Chlamydomonas. This key difference has allowed the generation of a number of mutants that are defective in flagellar assembly and/or function. These flagellar mutants have been useful in both classical genetic studies and complementation analysis in stable diploids. Complementation of a number of flagellar defects has also been observed in temporary quadriflagellate dikaryons formed by cell fusion during mating. For example, when cells with paralyzed flagella were mated with wild-type cells, the mutant flagella began to beat within a few minutes [2], [3], [4]. Moreover, the ease of isolation and purification of flagella from Chlamydomonas and the subsequent fractionation of flagella into its separate components has allowed not only the assignment of proteins to various structures that comprise the axoneme but also identification of the role they play in flagellar assembly and function [5].
The building block for flagella is the 9 + 2 microtubular axoneme. The basal body, a complex composed of triplet microtubules present at the base of each flagellum, serves as the template for nucleation of the 9 outer doublet microtubules. Localization of the basal bodies, and thus the flagella, within the cell is regulated by a complex fibrous structure that connects the basal bodies not only to each other but also to the nucleus and the rootlet microtubules of the cell body [6], [7]. Two singlet microtubules, known as the central pair, are present within the center of the axoneme. Dynein arms bind to the outer doublet microtubules and interact with adjacent doublet microtubules resulting in flagellar bending and thus motility. The outer doublet microtubules also provide the binding site for the radial spokes, a complex array of proteins that together with the central pair function to regulate flagellar motility [5].
Section snippets
Assembling a flagellum and intraflagellar transport
When flagella are removed from cells by mechanical shear or chemical stress, the cells immediately begin to grow new flagella [8]. This regenerative process occurs with deceleratory kinetics. The initial rate of flagellar elongation is rapid (∼0.4 μm/min) but as the flagella approach their pre-deflagellation length the rate of flagellar growth slows (∼0.15–0.2 μm/h) [8]. As the flagella elongate, axonemal precursors are incorporated at the distal tip (i.e., the plus or fast-growing end of the
Active control of flagellar length in Chlamydomonas
In 1969, Rosenbaum et al. made the seminal observation that Chlamydomonas dynamically monitors flagellar length thus ensuring equality between the two flagella [8]. This active regulation of flagellar length was demonstrated by the deflagellation of cells under conditions sufficient to remove only one flagellum from individual cells, resulting in a population of “long-zero” cells. The remaining flagellum of the long-zero cells immediately began to shorten to an intermediate length while the
Mutants of flagellar length control in Chlamydomonas
The most compelling evidence for the active control of flagellar length, however, is the behavior of mutants that can no longer regulate length [37], [38], [39]. To date, two classes of flagellar length mutants have been obtained. The first class includes six mutants with short flagella (shf) [38], [40], [41]. These shf mutants define three genes: SHF1 (three alleles), SHF2 (two alleles), and SHF3 (one allele). shf mutants have flagella that are of equal length but are only 6 μm long compared to
Genes involved in the regulation of flagellar length
Recently, a number of studies have begun to identify proteins that when mutated or their levels decreased by RNA interference (RNAi) have defects in assembling flagella of an appropriate length. Consistent with the pharmacological evidence that signal transduction plays a pivotal role in the regulation of flagellar length, the majority of these proteins are involved in signaling pathways.
Models for length control
A number of models have been proposed to explain the mechanisms used to control flagellar length. The simplest of these models is the limiting or quantal synthesis model, which proposes that one or more key components of the flagella are synthesized in a defined, limiting amount. Flagellar length therefore, would be directly proportional to the quantity of protein synthesized. Although there is evidence for the limited synthesis of a key axonemal component in sea urchins (i.e., tetkin A) [66],
Acknowledgements
We thank Drs. Dorothy Turetsky (OSU-CHS) and Pete Lefebvre (U MN) for helpful comments on the manuscript.
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