The corresponding oda16 and ida3 mutants assemble full-length flagella that specifically lack ODAs or IDAs I1/f, respectively, but of otherwise normal ultrastructure ( Ahmed and Mitchell, 2005 Hunter et al., 2018). With respect to axonemal proteins, IFT of outer dynein arms (ODAs) and the inner dynein arm (IDA) I1/f requires the adapter proteins ODA16 and IDA3, respectively ( Ahmed et al., 2008 Dai et al., 2018 Hunter et al., 2018). The octameric BBSome, for example, acts as a linker for a diverse group of transmembrane and membrane-associated proteins, attaching them indirectly to IFT trains ( Nachury et al., 2007 Liu and Lechtreck, 2018 Wingfield et al., 2018). However, many other proteins do not bind directly to the IFT trains but the interaction is mediated by IFT cargo adapters. Tubulin, the most abundant flagellar protein, binds directly to the N-terminal domains of the IFT-B core proteins IFT74 and IFT81 ( Bhogaraju et al., 2013 Craft et al., 2015 Kubo et al., 2016 Craft Van De Weghe et al., 2020). This raises the question how IFT trains, composed of just 22 IFT proteins and the associated kinesin-2 and IFT dynein motors, interact with the large number of diverse flagellar proteins. Numerous proteins of the flagellar axoneme, matrix, and membrane have been shown to use the IFT pathway for flagellar entry and exit ( Lechtreck, 2015). Protein transport into flagella involves intraflagellar transport (IFT), a motor-based bidirectional motility of protein carriers (i.e., ‘IFT trains’) ( Kozminski et al., 1993). ARMC2 belongs to a growing class of cargo-specific adapters that enable flagellar transport of preassembled axonemal substructures by IFT.Ĭilia 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). We conclude that ARMC2 is a cargo adapter required for IFT of radial spokes to ensure their assembly along flagella. In armc2/pf27 mutants, IFT of radial spokes was abolished and the presence of radial spokes was limited to the proximal region of flagella. After concomitant unloading at the flagellar tip, RSP3 attached to the axoneme whereas ARMC2 diffused back to the cell body. 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. Chlamydomonas ARMC2 was highly enriched in growing flagella and tagged ARMC2 and the spoke protein RSP3 co-migrated on anterograde trains. 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. 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.
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