RABL4/IFT27 in a nucleotide‐independent manner promotes phospholipase D ciliary retrieval via facilitating BBSome reassembly at the ciliary tip

Abstract

AbstractCertain ciliary transmembrane and membrane‐associated signaling proteins export from cilia as intraflagellar transport (IFT) cargoes in a BBSome‐dependent manner. Upon reaching the ciliary tip via anterograde IFT, the BBSome disassembles before being reassembled to form an intact entity for cargo phospholipase D (PLD) coupling. During this BBSome remodeling process, Chlamydomonas Rab‐like 4 GTPase IFT27, by binding its partner IFT25 to form the heterodimeric IFT25/27, is indispensable for BBSome reassembly. Here, we show that IFT27 binds IFT25 in an IFT27 nucleotide‐independent manner. IFT25/27 and the IFT subcomplexes IFT‐A and ‐B are irrelevant for maintaining the stability of one another. GTP‐loading onto IFT27 enhances the IFT25/27 affinity for binding to the IFT‐B subcomplex core IFT‐B1 entity in cytoplasm, while GDP‐bound IFT27 does not prevent IFT25/27 from entering and cycling through cilia by integrating into IFT‐B1. Upon at the ciliary tip, IFT25/27 cycles on and off IFT‐B1 and this process is irrelevant with the nucleotide state of IFT27. During BBSome remodeling at the ciliary tip, IFT25/27 promotes BBSome reassembly independent of IFT27 nucleotide state, making postremodeled BBSomes available for PLD to interact with. Thus, IFT25/27 facilitates BBSome‐dependent PLD export from cilia via controlling availability of intact BBSomes at the ciliary tip, while IFT27 nucleotide state does not participate in this regulatory event.