Abstract
Background
The cilium is a microtubule-based organelle essential for cellular signaling, whose assembly depends on intraflagellar transport (IFT). IFT20, a unique IFT-B component, localizes to both the Golgi apparatus and cilium/flagellum in mammalian systems and plays a role in ciliary membrane protein trafficking. Here, we analyzed IFT20-mediated ciliary trafficking of channelrhodopsin-1 (ChR1) in Chlamydomonas reinhardtii.Methods and results
Co-immunocytochemistry showed that IFT20 and ChR1 co-localized throughout the flagella of wild-type cells. Using fla8 (kinesin-2) and dhc1b-3 (dynein) mutants, we found this co-localization to be motor-dependent, with disrupted anterograde and retrograde transport causing protein stagnation near the basal body and ciliary tip, respectively. In the bbs1 mutant, IFT20 is distributed along the flagellar length and basal body, whereas ChR1 is restricted to the flagella only. Further, protein interaction network analysis reveals that IFT20 serves as a central adaptor, interfacing ancillary ciliary trafficking components, including CrARL11 (Arf), the IFT complex, and BBSome subunits. The CrARL11 (Arf) co-localized with IFT20 in the flagella of the wild-type strain, suggesting a potential interaction. Fluorescence spectroscopy shows that upon GTP binding, IFT20 undergoes concentration-dependent fluorescence quenching. Far-UV CD spectroscopy revealed that recombinant IFT20 has a predominantly helical structure, with modest spectral shifts upon GTP addition; notably, IFT20 lacks a canonical GTPase switch region or Ras-like G-domain, indicating an atypical mode of GTP interaction.Conclusions
These findings extend the mechanistic understanding of ciliary membrane protein delivery in C. reinhardtii and indicate that IFT20-mediated rhodopsin trafficking reflects a conserved transport mechanism across lower eukaryotes.