Fig. 3.

Overview of IFT adapter–cargo interactions. Diagram summarizing the interactions between IFT proteins, adapters and their cargoes. Tubulin dimers bind directly to IFT-B via IFT74 and IFT81. The BBSome associates with the IFT-B complex via IFT25–IFT27 (IFT25/27), and TULP3 associates with the IFT-A complex, apparently via IFT122. Both are multivalent adapters participating in the transport of diverse transmembrane and membrane-associated proteins, including GPCRs, ion channels and single-pass transmembrane (TM) proteins. The ciliary targeting sequences (CTSs; turquoise) of BBSome and TULP3 cargoes are also the adapter-binding sites. Ubiquitylation of some GPCRs (such as SMO and SSTR3) is required for BBSome-dependent export from cilia. The IFT complexes and the BBSome lack known ubiquitin-binding sites, and the interaction between the BBSome and ubiquitin sidechains may involve TOM1L2 as an additional adapter. The known adapters for axonemal substructures each have a single cargo specificity. For ODA16, the binding partner on the IFT trains is the N-terminal domain of IFT46, and IFT56 is reported to be the IFT binding partner for IDAs. RSP3 is the only radial spoke (RS) protein expressed in Drosophila auditory neurons, which form cilia by the IFT pathway, suggesting that it could be the binding partner of ARMC2.