Effect of the N-Terminal Helix and Nucleotide Loading on the Membrane and Effector Binding of Arl2/3.

The small GTP-binding proteins Arl2 and Arl3, which are close homologs, share a number of interacting partners and act as displacement factors for prenylated and myristoylated cargo. Nevertheless, both proteins have distinct biological functions. Whereas Arl3 is considered a ciliary protein, Arl2 has been reported to be involved in tubulin folding, mitochondrial function, and Ras signaling. How these different roles are attained by the two homolog proteins is not fully understood. Recently, we showed that the N-terminal amphipathic helix of Arl3, but not that of Arl2, regulates the release of myristoylated ciliary proteins from the GDI-like solubilizing factor UNC119a/b. In the biophysical study presented here, both proteins are shown to exhibit a preferential localization and clustering in liquid-disordered domains of phase-separated membranes. However, the membrane interaction behavior differs significantly between both proteins with regard to their nucleotide loading. Whereas Arl3 and other Arf proteins with an N-terminal amphipathic helix require GTP loading for the interaction with membranes, Arl2 binds to membranes in a nucleotide-independent manner. In contrast to Arl2, the N-terminal helix of Arl3 increases the binding affinity to UNC119a. Furthermore, UNC119a impedes membrane binding of Arl3, but not of Arl2. Taken together, these results suggest an interplay among the nucleotide status of Arl3, the location of the N-terminal helix, membrane fluidity and binding, and the release of lipid modified cargos from carriers such as UNC119a. Since a specific Arl3-GEF is postulated to reside inside cilia, the N-terminal helix of Arl3•GTP would be available for allosteric regulation of UNC119a cargo release only inside cilia.

Structural basis for Arl3-specific release of myristoylated ciliary cargo from UNC119.

Access to the ciliary membrane for trans-membrane or membrane-associated proteins is a regulated process. Previously, we have shown that the closely homologous small G proteins Arl2 and Arl3 allosterically regulate prenylated cargo release from PDEδ. UNC119/HRG4 is responsible for ciliary delivery of myristoylated cargo. Here, we show that although Arl3 and Arl2 bind UNC119 with similar affinities, only Arl3 allosterically displaces cargo by accelerating its release by three orders of magnitude. Crystal structures of Arl3 and Arl2 in complex with UNC119a reveal the molecular basis of specificity. Contrary to previous structures of GTP-bound Arf subfamily proteins, the N-terminal amphipathic helix of Arl3·GppNHp is not displaced by the interswitch toggle but remains bound on the surface of the protein. Opposite to the mechanism of cargo release on PDEδ, this induces a widening of the myristoyl binding pocket. This leads us to propose that ciliary targeting of myristoylated proteins is not only dependent on nucleotide status but also on the cellular localization of Arl3.

The GDI-like solubilizing factor PDEδ sustains the spatial organization and signalling of Ras family proteins.

We identify a role for the GDI-like solubilizing factor (GSF) PDEδ in modulating signalling through Ras family G proteins by sustaining their dynamic distribution in cellular membranes. We show that the GDI-like pocket of PDEδ binds and solubilizes farnesylated Ras proteins, thereby enhancing their diffusion in the cytoplasm. This mechanism allows more effective trapping of depalmitoylated Ras proteins at the Golgi and polycationic Ras proteins at the plasma membrane to counter the entropic tendency to distribute these proteins over all intracellular membranes. Thus, PDEδ activity augments K/Hras signalling by enriching Ras at the plasma membrane; conversely, PDEδ down-modulation randomizes Ras distributions to all membranes in the cell and suppresses regulated signalling through wild-type Ras and also constitutive oncogenic Ras signalling in cancer cells. Our findings link the activity of PDEδ in determining Ras protein topography to Ras-dependent signalling.

Arl2-GTP and Arl3-GTP regulate a GDI-like transport system for farnesylated cargo.

Lipidated Rho and Rab GTP-binding proteins are transported between membranes in complex with solubilizing factors called 'guanine nucleotide dissociation inhibitors' (GDIs). Unloading from GDIs using GDI displacement factors (GDFs) has been proposed but remains mechanistically elusive. PDEδ is a putative solubilizing factor for several prenylated Ras-subfamily proteins. Here we report the structure of fully modified farnesylated Rheb-GDP in complex with PDEδ. The structure explains the nucleotide-independent binding of Rheb to PDEδ and the relaxed specificity of PDEδ. We demonstrate that the G proteins Arl2 and Arl3 act in a GTP-dependent manner as allosteric release factors for farnesylated cargo. We thus describe a new transport system for farnesylated G proteins involving a GDI-like molecule and an unequivocal GDF. Considering the importance of PDEδ for proper Ras and Rheb signaling, this study is instrumental in developing a new target for anticancer therapy.

The structure of an Arf-ArfGAP complex reveals a Ca2+ regulatory mechanism.

Arfs are small G proteins that have a key role in vesicle trafficking and cytoskeletal remodeling. ArfGAP proteins stimulate Arf intrinsic GTP hydrolysis by a mechanism that is still unresolved. Using a fusion construct we solved the structure of the ArfGAP ASAP3 in complex with Arf6 in the transition state. This structure clarifies the ArfGAP catalytic mechanism and shows a glutamine((Arf6)) and an arginine finger((ASAP3)) as the important catalytic residues. Unexpectedly the structure shows a calcium ion, liganded by both proteins in the complex interface, stabilizing the interaction and orienting the catalytic machinery. Calcium stimulates the GAP activity of ASAPs, but not other members of the ArfGAP family. This type of regulation is unique for GAPs and any other calcium-regulated processes and hints at a crosstalk between Ca(2+) and Arf signaling.

Post-proteomic identification of a novel phage-encoded streptodornase, Sda1, in invasive M1T1 Streptococcus pyogenes.

The M1T1 strain remains the most frequently isolated strain from group A streptococcal (GAS) infection cases worldwide. We previously reported that M1T1 differs from the fully sequenced M1 SF370 strain. To better understand the reason for the persistence and increased virulence of M1T1, we analysed its secreted proteome and identified two virulence proteins that are not present in the sequenced M1 SF370 strain: streptococcal pyrogenic exotoxin A (SpeA) and a streptodornase D (SdaD) homologue. In the present study, we determined the nucleotide sequence of the M1T1 streptodornase and found that its deduced amino acid sequence is highly similar to other streptococcal streptodornases, and is most closely related to the SdaD of GAS strain M49. M1T1 Sda shares two highly conserved domains with several DNases and putative DNases in streptococci; however, it possesses a unique C-terminal amino acid sequence. Thus, we named the protein Sda1, and we detected the presence of the sda1 gene in 16 M1T1 clinical isolates. The cloned and expressed Sda1 degrades both streptococcal and mammalian DNA at physiological pH. Amino acid similarity analyses of known GAS deoxyribonucleases suggest that Sda1 may be a chimeric protein created through recombination events. Moreover, a natural mutation that resulted in longer Sda1 and SdaD as compared to other GAS DNases was found to confer increased activity on the protein. Analysis of the sequences flanking sda1 determined that it is carried by a prophage or a prophage-like element inserted in the tRNA-Ser gene of M1T1 GAS. Ongoing studies in our laboratory aim to determine the contribution of Sda1 to the virulence of this globally disseminated M1T1 strain.