Videos of Chlamydomonas "paralyzed flagella" movement!
A variety of paralyzed flagella mutants have been identified in the haploid algae, Chlamydomonas reinhardtii. Among those that do not swim, many are missing components of the radial-spokes/central-pair complex. Since these mutants have the full wild-type complement of dynein ATPases necessary for movement, it was unclear why they are paralyzed. Indeed, a class of suppressor mutations allows flagellar bending without restoring the missing radial-spokes/central-pair structures. In addition, naturally occurring axonemes missing the central-pair microtubules are capable of bending. Thus why are mutants missing the radial-spokes/central-pair complex paralyzed?
In fact, pf mutants missing the central-pair microtubules (pf18), radial spokes (pf14), or the radial spoke heads (pf17) can move under different nucleotide conditions 2. These conditions are low [ATP], a mixture of ATP and ADP, or a mixture of ATP and ribose-modified ATP analogs. Erica Frey, a Howard Hughes Undergraduate student in my laboratory, surveyed all known pf mutants that are paralyzed as intact cells to determine whether mutants missing components other than the central-pair/radial-spoke complex can also move in a nucleotide-dependent manner. This work is now published.
The following are examples of movement of pf mutants captured using NIH Image and animated using GifbuilderTM.
Each are ~ 10k in size.
pf15 axonemes are missing the central pair of microtubules.
pf16 axonemes have an unstable central pair microtubule.
pf5 axonemes are missing components of the radial spokes and inner dynein arms.
The defect(s) in pf12 axonemes are essentially uncharacterized.
The following hypothesis was proposed to explain nucleotide-dependent movement of these pf mutants. There is a nucleotide binding site distinct from the mechanochemical active site responsible for regulation. Under normal physiological ATP concentration, the regulatory nucleotide binding site is occupied by ATP and inhibits the action of dynein. The central-pair/radial-spoke complex overcomes this ATP inhibition in a systematic fashion to allow bend formation and propagation. Pf mutants missing the central-pair/radial-spoke complex cannot overcome this ATP inhibition. By altering nucleotide conditions, either by reducing ATP concentration, or adding millimolar concentrations of ADP, the regulatory nucleotide binding site is unoccupied or occupied by ADP, thus releasing ATP inhibition. The same nucleotide conditions slow the mechanochemical cycle sufficiently to produce regular movement without the radial-spoke/central-pair complex.