Protein Particles inChlamydomonasFlagella Undergo a Transport Cycle Consisting of Four Phases

Abstract

We used an improved procedure to analyze the intraflagellar transport (IFT) of protein particles in Chlamydomonas and found that the frequency of the particles, not only the velocity, changes at each end of the flagella. Thus, particles undergo structural remodeling at both flagellar locations. Therefore, we propose that the IFT consists of a cycle composed of at least four phases: phases II and IV, in which particles undergo anterograde and retrograde transport, respectively, and phases I and III, in which particles are remodeled/exchanged at the proximal and distal end of the flagellum, respectively. In support of our model, we also identified 13 distinct mutants of flagellar assembly (fla), each defective in one or two consecutive phases of the IFT cycle. The phase I-II mutant fla10-1 revealed that cytoplasmic dynein requires the function of kinesin II to participate in the cycle. Phase I and II mutants accumulate complex A, a particle component, near the basal bodies. In contrast, phase III and IV mutants accumulate complex B, a second particle component, in flagellar bulges. Thus, fla mutations affect the function of each complex at different phases of the cycle.

A healthy understanding of intraflagellar transport

“…In support of this hypothesis are recent results by Iomini et al [2001] who have divided IFT into four subclasses (Phases I-IV) based on an analysis not only of the velocity of particle movement but also of the frequency of particle movement. Phases II and IV correspond to anterograde and retrograde movements, respectively, as originally defined by Kozminski et al [1993].…”

“…Phases I and III refer to events that occur at the proximal and distal ends of the flagellum. Iomini et al [2001] proposed that during Phases I and III, IFT components become remodeled and/or exchanged at either end of the flagellum. Phases I and III could be components of transcriptional or signal transduction control mechanisms.…”

Dynein and Intraflagellar Transport

“…Independently, another group also cloned Chlamydomonas DHC1b and identified dhc1b null mutants, which have flagella assembly defects and accumulate particles in their extremely short flagella (Porter et al, 1999). In addition to null mutants in DHC1b, three Chlamydomonas temperature-sensitive dhc1b mutants have been identified (Iomini et al, 2001;Engel et al, 2012;Lin et al, 2013). While one of them, dhc1b-2, has half-length flagella at permissive temperature, two of them, dhc1b-3 and fla24, have normal-length flagella at permissive temperature yet have a reduced level of DHC1b in the flagella and reduced retrograde IFT.…”

Sperm dysfunction and ciliopathy

“…Mice with deletions of the gene encoding another IFT-B component, IFT88/Polaris, develop PKD, hepatic fibrosis, defects in photoreceptor assembly and maintenance, and situs inversus [98,99]. Mutants lacking some IFT-A components have flagella with bulged membrane at the tips in Chlamydomonas [100,101]. Mutations of IFT-A components in mouse show a phenotype similar to bulged cilia [102].…”