Abstract
Intraflagellar transport (IFT) carries proteins into flagella but how IFT trains interact with the large number of diverse proteins required to assemble flagella remains largely unknown. Here, we show that IFT of radial spokes in Chlamydomonas requires ARMC2/PF27, a conserved armadillo repeat protein associated with male infertility and reduced lung function. Chlamydomonas ARMC2 was highly enriched in growing flagella and tagged ARMC2 and the spoke protein RSP3 co-migrated on anterograde trains. In contrast, a cargo and an adapter of inner and outer dynein arms moved independently of ARMC2, indicating that unrelated cargoes distribute stochastically onto the IFT trains. After concomitant unloading at the flagellar tip, RSP3 attached to the axoneme whereas ARMC2 diffused back to the cell body. In armc2/pf27 mutants, IFT of radial spokes was abolished and the presence of radial spokes was limited to the proximal region of flagella. We conclude that ARMC2 is a cargo adapter required for IFT of radial spokes to ensure their assembly along flagella. ARMC2 belongs to a growing class of cargo-specific adapters that enable flagellar transport of preassembled axonemal substructures by IFT.
Highlights
Cilia and eukaryotic flagella consist of hundreds of distinct proteins, which are synthesized in the cell body and moved posttranslationally into the organelle (ROSENBAUM AND CHILD 1967; PAZOUR et al 2005)
Intraflagellar transport (IFT) of tagged radial spoke (RS) was not observed in armc2 mutants, and 3. tagged ARMC2 and the essential RS protein RSP3 typically co-migrated on anterograde
In comparison to wild-type flagella, the phosphorylation of several spoke proteins is reduced in pf27/armc2 (HUANG et al 1981)
Summary
Cilia and eukaryotic flagella consist of hundreds of distinct proteins, which are synthesized in the cell body and moved posttranslationally into the organelle (ROSENBAUM AND CHILD 1967; PAZOUR et al 2005). RS assembly in the proximal region of pf flagella could result from residual entry of RSs by diffusion followed by binding to the nearest available docking sites Such a scenario could explain why the phosphorylation state of several RS proteins is altered in pf as it has been proposed that phosphorylation of these proteins occurs near the flagellar tip, which the RSs would fail to reach in pf (HUANG et al 1981; YANG AND YANG 2006; GUPTA et al 2012). IFT of ODAs, IDAs I1/f and RSs, three large axonemal substructures pre-assembled in the cell body, requires adapters with single cargo-specificity. As these adapters enter flagella and bind to IFT in the absence of their respective cargoes, we propose that the regulation of IFT-adapter interaction is a critical step for controlling cargo import into flagella by IFT
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