ARL16

ARL16 is an ancient ARF-family GTPase that functions in two distinct cellular contexts: innate immune regulation and ciliary trafficking (PMID:34247240). In its GTP-bound form, ARL16 binds directly to the C-terminal domain of the cytosolic RNA sensor RIG-I, blocking RIG-I–RNA association and thereby suppressing IFN-β signaling and antiviral responses (PMID:21233210). At the Golgi, ARL16 is required for a specific trafficking pathway that delivers IFT140 and INPP5E to cilia; its deletion causes accumulation of these cargoes at the Golgi, reduces ciliogenesis, and depletes ARL13B and ARL3 from cilia, and GTP-locked ARL16 rescues equivalent defects caused by loss of the ARF GAPs ELMOD1/3, placing ARL16 downstream of these regulators (PMID:35196065, PMID:34818063).

1. 2011 High

   The first functional role for ARL16 was established as a GTP-dependent negative regulator of innate immunity, resolving how a small GTPase could directly modulate RIG-I signaling by competing with RNA binding at RIG-I's C-terminal domain.

   Evidence Co-immunoprecipitation with domain-mapping and GTP-binding mutants, RNAi knockdown with IFN-β reporter assays in cultured cells

   PMID:21233210

   Open questions at the time

   • Endogenous stoichiometry and physiological conditions under which ARL16 inhibits RIG-I are not defined
   • No GAP or GEF identified for ARL16 in the RIG-I context
   • Whether ARL16's immune and ciliary functions are coordinated or independent is unknown
2. 2021 Medium

   Phylogenetic analysis established ARL16 as an ancient member of the ARF GTPase family present in the last eukaryotic common ancestor, suggesting deeply conserved functions beyond mammalian immunity.

   Evidence Molecular phylogenetics across 114 eukaryotic species with manual curation of >2,000 ARF family sequences

   PMID:34247240

   Open questions at the time

   • No functional data accompany the phylogenetic assignment
   • Whether ancestral ARL16 function relates to cilia or immunity is unresolved
3. 2022 High

   ARL16 was shown to operate at the Golgi in a specific trafficking pathway required for ciliary delivery of IFT140 and INPP5E, and epistasis experiments placed it downstream of the ARF GAPs ELMOD1/3, establishing a second major cellular role for this GTPase.

   Evidence CRISPR/Cas9 knockout in MEFs with immunofluorescence, subcellular fractionation, ciliary measurements; GTP-locked ARL16 mutant rescue of ELMOD1/3 KO phenotypes

   PMID:34818063 PMID:35196065

   Open questions at the time

   • Direct effectors of ARL16 at the Golgi are unknown
   • How ARL16 activation is regulated by ELMOD1/3 (direct substrate vs. indirect) is not resolved
   • Mechanism by which ARL16 loss increases ciliary length while reducing ciliogenesis is unexplained

• No GEF or GAP specific to ARL16 has been biochemically identified, no structural model of ARL16 in complex with any partner exists, and whether the immune-regulatory and ciliary trafficking functions involve shared or distinct pools of ARL16 remains open.
• No GEF identified for ARL16
• No crystal or cryo-EM structure available
• In vivo phenotyping in whole organisms (mouse knockout) has not been reported