IFT22

IFT22 (RABL5) is a Rab-like small GTPase that functions as a subunit of the intraflagellar transport complex B (IFT-B) core, embedded among IFT88, IFT81, IFT74/72, IFT52, IFT46, IFT27, and IFT25 (PMID:20435895). Beyond its structural role in IFT-B, IFT22 governs the cytoplasmic pool size of both IFT-A and IFT-B particles: its depletion reduces the cellular reservoir of IFT proteins and causes their abnormal accumulation within flagella, a phenotype mirrored by overexpression (PMID:22076686). IFT22 is an active GTPase with low intrinsic activity that, in its GTP-bound state and together with the GTP-bound Arf-like GTPase BBS3/ARL6, recruits the BBSome to the basal body for coupling with the IFT-B1 subcomplex and ciliary entry; the BBSome is then handed off to IFT trains at the ciliary base, since IFT22 is not required for BBSome movement within cilia (PMID:31953262). Consistent with this targeting role, IFT22 is dispensable for ciliogenesis and ciliary protein trafficking per se in mammalian cells (PMID:29654116). No disease association is established within this corpus.

1. 2010 High

   Established that IFT22 is a defined architectural component of the IFT-B core rather than a peripheral associate, anchoring it within the conserved transport machinery.

   Evidence Biochemical fractionation, yeast two-hybrid, bacterial coexpression and chemical cross-linking of the IFT-B core in Chlamydomonas

   PMID:20435895

   Open questions at the time

   • Did not define IFT22's direct binding partners within the core
   • No functional role assigned
2. 2012 Medium

   Showed that IFT22 sets the cytoplasmic pool size of IFT particles, distinguishing a regulatory function from passive cargo transport.

   Evidence RNAi depletion, overexpression, Western blotting and fluorescence imaging in C. reinhardtii

   PMID:22076686

   Open questions at the time

   • Mechanism linking pool-size control to GTPase activity not defined
   • Single lab, single organism
3. 2014 Medium

   Placed IFT22 functionally upstream in directional intracellular transport via genetic interaction with BBS7-mediated retrograde melanosome movement.

   Evidence Morpholino double-knockdown epistasis with melanosome transport imaging in zebrafish

   PMID:24938409

   Open questions at the time

   • Morpholino knockdown without genetic mutant confirmation
   • Molecular basis of the IFT22–BBS7 interaction not resolved
4. 2018 Medium

   Demonstrated that IFT22 is dispensable for ciliogenesis and ciliary protein trafficking, refocusing its essential function away from core IFT delivery.

   Evidence CRISPR/Cas9 knockout with ciliogenesis and ciliary trafficking assays in mammalian cells

   PMID:29654116

   Open questions at the time

   • Negative result does not identify the process that requires IFT22
   • Single cell-type readout
5. 2020 High

   Resolved IFT22's specific function as a GTP-state-dependent BBSome-recruiting factor acting with BBS3/ARL6 at the basal body, unifying the prior structural and pool-size observations.

   Evidence In vitro GTPase assays, reciprocal pulldowns, single-particle in vivo imaging and mutant rescue in Chlamydomonas

   PMID:31953262

   Open questions at the time

   • Structural basis of the IFT22–BBS3–BBSome assembly not determined
   • Nucleotide cycling regulators (GEF/GAP) for IFT22 unknown

• How IFT22 nucleotide state is regulated and how the IFT22/BBS3-bound BBSome is physically transferred onto IFT trains remains unresolved.
• No GEF/GAP identified for IFT22
• No structural model of the handoff to IFT-B1 trains
• Mammalian relevance of the BBSome-recruitment mechanism not tested