ARF-Like (ARL) Proteins - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 227

underlying the cytokinesis defects observed in ARL3-depleted mammalian cells,
but further investigations are required to test such a model.
Better understood, or perhaps just more thoroughly studied, is the role of ARL3
in the regulation of ciliary processes, including ciliogenesis and cilia signaling. A
phylogenetic study found that ARL3 was present only in ciliated organisms
throughout eukaryotic evolution and this provides strong evidence for its role in
that organelle (Avidor-Reiss et al. 2004 ). In a study of the ARL3 ortholog of the
protozoan Leishmania donovani ( Ld ARL-3A), it was shown that expression of a
constitutively active mutant, Ld ARL-3[Q70L], resulted in decreased flagellum
length in the extracellular promastigotes (Cuvillier et al. 2000 ). It was later
shown that ARL3 localizes to the connecting cilium of human photoreceptor cells
and the primary cilium of cultured mouse embryonic fibroblast (NIH3T3) cells
(Grayson et al. 2002 ; Zhou et al. 2006 ). A functional link to ciliary processes was
established with the development of an ARL3-null mouse model (Schrick
et al. 2006 ) as these mice exhibit several phenotypes that were typical of cilia
defects, including cyst formation in the kidney, liver, and pancreas, and impaired
photoreceptor development. Mice lacking ARL3 also had severe physical deformi-
ties, and all died within three weeks of birth. Interestingly, cilia in the ARL3-null
mice appeared to be structurally normal, which suggests that the observed pheno-
types are a result of impairment(s) in cilia function rather than in ciliogenesis. This
conclusion was further supported by studies of ARL3 in C. elegans (Li et al. 2010 )
in which ARL3-null worms displayed normally structured cilia. However, similar
to the study in Leishmania , expression of the constitutively active mutant, ARL3
[Q72L], resulted in impaired ciliogenesis. In a fascinating finding that is likely to
lead to real insights in some ARL functions, depletion of ARL3 partially rescued
the ciliogenesis defects that result from deletion of the worm ARL13 gene
(Li et al. 2010 ). These data provide consistent evidence for the role of ARL3 as a
negative regulator of ciliogenesis and provide intriguing functional links between
ARL3 and ARL13.
Though not required for cilia formation, ARL3 plays pivotal roles in the
regulation of cilia signaling. In C. elegans , ARL3 acting in concert with ARL13
is required for the proper regulation of intraflagellar transport through HDAC6-
mediated regulation of the association of subcomplex B with the kinesin motor,
KIF17 (Li et al. 2010 ). Through interaction with specific effectors, including PDE ʴ

and HRG4 (also named UNC119), ARL3 is also involved in the targeting of lipid-
modified cargos to the primary cilium (Ismail et al. 2011 , 2012 ; Wright et al. 2011 ;
Watzlich et al. 2013 ). A yeast two-hybrid screen initially identified ARL3 as a
binding partner of PDE

is responsible for the membrane
targeting of a subset of prenylated cargo to the primary cilium (Zhang et al. 2012 )
with ARL3 functioning as a release factor for the farnesylated cargo (Ismail
et al. 2011 ; Watzlich et al. 2013 ). This role is further supported by the involvement
of the Retinitis Pigmentosa GTPase Regulator (RPGR) in the PDE

ʴ (Linari et al. 1999 ). PDE ʴ

/cargo/ARL3
mechanism (Watzlich et al. 2013 ). RPGR localizes to the cilium where it regulates
ciliary traffic (Brunner et al. 2010 ). RPGR binds PDE

ʴ

directly and is proposed to
serve as a scaffolding protein to facilitate ARL3-mediated release of cargo from

ʴ