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

Abstract

Certain 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.

RABL2 promotes the outward transition zone passage of signaling proteins in cilia via ARL3

“…S2 C and D , and Movies S1 and S2 ) ( 47 ). Chlamydomonas mutants lacking even a single IFT-A subunit accumulate certain proteins in cilia as their retrograde IFT is dampened due to the loss of IFT-A ( 48 , 49 ). RABL2 accumulated in cilia of the IFT-A subunit mutant ift43 ( SI Appendix, Fig.…”

Organization, functions, and mechanisms of the BBSome in development, ciliopathies, and beyond

“…The ARL6 GEF activity of IFT27 should also be considered with caution, considering that the GEF activity of IFT27 in vitro is low and the biochemical assays are performed on whole cell samples rather than cilia alone ( Liew et al, 2014 ). In C. reinhardtii , IFT25/27 does not depend on IFT27’s nucleotide state to cycle on and off IFT-B and fails to activate ARL6 in vitro , arguing against a GEF function for IFT27 ( Liu et al, 2023a ; Liu et al, 2023b ). Similar to IFT27, IFT25 depletion also has no effect on ciliary entry of the BBSome, but it does impair BBSome ciliary exit ( Dong et al, 2017 ).…”

The emerging functions of intraflagellar transport 52 in ciliary transport and ciliopathies

“…35 Similarly, the IFT25-IFT27 complex helps in reassembly of BBSome at the ciliary tip, thereby enhancing the interaction of IFT-A with BBSome during the retrograde transport of proteins such as phospholipase D (PLD). 36 This process requires the interaction of IFT27 with BBS3 (also known as ARL6). 37 ARL13 also plays a crucial role in IFT-A/ BBSome-mediated transport of PLD at the transition zone of proximal ciliary region.…”

“…It is noted that the mutually exclusive binding of IFT25‐IFT27 complex or RABL2 to the IFT74/BBS22‐IFT81 helps in loading the IFT‐B complex onto the BBSome at the ciliary base during anterograde transport 35 . Similarly, the IFT25‐IFT27 complex helps in reassembly of BBSome at the ciliary tip, thereby enhancing the interaction of IFT‐A with BBSome during the retrograde transport of proteins such as phospholipase D (PLD) 36 . This process requires the interaction of IFT27 with BBS3 (also known as ARL6) 37 .…”