LIG_ARL_BART_1

Small G-proteins are involved in regulation of a variety of cellular processes and act as molecular switches cycling between GDP- and GTP-bound states. ARF subfamily proteins are mainly crucial to regulation of vesicular transport and undergo so-called beta-register shift between the GDP and the GTP conformation that promotes the release of an ARF-specific N-terminal myristoylated alpha-helix (Hanzal-Bayer,2002). The ARF-like (ARL) proteins are more heterogeneous than ARFs, and have more diverse functions, but nevertheless they share the same signaling mechanism: during the GDP-GTP exchange ARL2 and ARL3 undergo a beta-sheet register shift similar to that of the ARFs. In contrast with ARFs, the N-terminal region is non-myristoylated for ARL2, and acetylated for ARL3, but both form alpha-helices that can adopt different conformations in complexes with different effectors of the two small GTPases.

The GTP-dependent interaction between ARL2 and its binding partner BART (ARL2BP) involves two interfaces, one of which is mediated mostly by the switch I region of ARL2, and the other one is ensured by a well-conserved sequence motif LLxILxx[LM] for the N-terminal helix of ARL2. The helix binds the hydrophobic pocket of the BART protein, formed by several BART alpha helices, and the residues L3, L4, I6 and L7 of ARL2 make an extensive hydrophobic contact with several BART residues. Additionally, several hydrogen bounds are formed at this interaction interface (Zhang,2009, 3DOE).

A similar binding pattern is revealed for ARL3-GTP in complex with its effector CCDC104/CFAR36, also called BARTL1 for its structural homology with BART. The interaction again involves two areas, both essential for the binding. The first area interface is formed predominantly by the switch I and II regions of ARL3. In the second interface the N-terminal amphipathic helix of ARL3, containing LLxILxx[LM] motif, is completely buried in the hydrophobic groove of BARTL1. Interestingly, the N-terminal helix of ARL3 is essential not only for the interaction with BARTL1, but for the ciliary localization of the small GTPase itself (Lokaj,2015).

This binding mode sets BART and BARTL1 apart from other effectors of ARL2/3, as well as other effectors of the Ras superfamily proteins, mainly because it directly involves the N-terminal switch. In complex with PDEd, the N-terminal helix of ARL2-GTP protrudes on the surface and has no interaction to either ARL2 or its effector protein (Hanzal-Bayer,2002). Based on this, the N-terminal helix of ARL2 in complex with Unc119 is predicted to be exposed to the solvent as well. Strikingly, the structure ARL3-GppNHp in complex with Unc119a shows that, although the typical beta-sheet register shift also takes place upon the nucleotide exchange in ARL3, the hydrophobic pocket on the surface of the small GTPase remains free to be occupied by its N-terminal helix, which remains at least partially attached to the surface, the location resembling that of the GDP-bound conformation of ARL3 (1FZQ) as well as other ARF/ARL subfamily proteins. It was also shown that the presence of the ARL3 helix is indispensible (but not sufficient) for the myristoylated cargo release from Unc119a (Ismail,2012).

Structural and/or functional defects of some ARLs may drive ciliopathies, as several of this subfamily proteins were shown to be involved in ciliogenesis and cilia maintenance. Namely, Retinitis Pigmentosa is associated with mutations in ARL3 (22884633, Kuhnel,2006). In addition, ARL2 and ARL3 are known to regulate various microtubule-dependent processes, such as tubulin dynamics and folding (Veltel,2008). The complex ARL2-GTP-BART was reported to enter mitochondria and to bind to ANT-1, an inner mitochondrial transmembrane protein responsible for antiport of ADP/ATP (Sharer,2002). Recent studies show that BART is essential for nuclear retention of STAT3 protein, and its activity is enhanced by the presence of ARL2 (Muromoto,2008).