Purification and characterization of GIT family of ADP-ribosylation factor (ARF) GTPase-activating proteins

A Method to Generate and Analyze Modified Myristoylated Proteins

Covalent lipid modification of proteins is essential to their cellular localizations and functions. Engineered lipid motifs, coupled with bio-orthogonal chemistry, have been utilized to identify myristoylated or palmitoylated proteins in cells. However, whether modified proteins have similar properties as endogenous ones has not been well investigated mainly due to lack of methods to generate and analyze purified proteins. We have developed a method that utilizes metabolic interference and mass spectrometry to produce and analyze modified, myristoylated small GTPase ADP-ribosylation factor 1 (Arf1). The capacities of these recombinant proteins to bind liposomes and load and hydrolyze GTP were measured and compared with the unmodified myristoylated Arf1. The ketone-modified myristoylated Arf1 could be further labeled by fluorophore-coupled hydrazine and subsequently visualized through fluorescence imaging. This methodology provides an effective model system to characterize lipid-modified proteins with additional functions before applying them to cellular systems.

Structure-activity relationship studies of QS11, a small molecule Wnt synergistic agonist

Both the Wnt/β-catenin signaling pathway and small GTPases of the ADP-ribosylation factors (ARF) family play important roles in regulating cell development, homeostasis and fate. The previous report of QS11, a small molecule Wnt synergist that binds to ARF GTPase-activating protein 1 (ARFGAP1), suggests a role for ARFGAP1 in the Wnt/β-catenin pathway. However, direct inhibition of enzymatic activity of ARFGAP1 by QS11 has not been established. Whether ARFGAP1 is the only target that contributes to QS11's Wnt synergy is also not clear. Here we present structure-activity relationship (SAR) studies of QS11 analogs in two assays: direct inhibition of enzymatic activity of purified ARFGAP1 protein and cellular activation of the Wnt/β-catenin pathway. The results confirm the direct inhibition of ARFGAP1 by QS11, and also suggest the presence of other potential cellular targets of QS11.

G-protein-coupled receptor kinase interacting protein-1 mediates intima formation by regulating vascular smooth muscle proliferation, apoptosis, and migration

OBJECTIVE

The G-protein-coupled receptor kinase interacting protein-1 (GIT1) is a scaffold protein that is important for phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling induced by angiotensin II and epidermal growth factor. Because GIT1 regulates signaling by several vascular smooth muscle cell (VSMC) growth factors, we hypothesized that intima formation would be inhibited by GIT1 depletion.

APPROACH AND RESULTS

Complete carotid ligation was performed on GIT1 wild-type and knockout (KO) mice. We compared changes between GIT1 wild-type and KO mice in carotid vascular remodeling, VSMC proliferation, and apoptosis in vivo and in vitro. Our data demonstrated that GIT1 deficiency significantly decreased intima formation after carotid ligation as a result of both reduced VSMC proliferation and enhanced apoptosis. To confirm the effects of GIT1 in vitro, we performed proliferation and apoptosis assays in VSMC. In mouse aortic smooth muscle cells (MASM), we found that the growth rate and [3H]-thymidine incorporation of the GIT1 KO MASM were significantly decreased compared with the wild-type MASM. Cyclin D1, which is a key cell cycle regulator, was significantly decreased in GIT1 KO cells. Serum deprivation of GIT1 KO MASM increased apoptosis 3-fold compared with wild-type MASM. Treatment of rat aortic smooth muscle cells with GIT1 small interfering RNA impaired cell migration. Both phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling were required for GIT1-dependent VSMC proliferation and migration, whereas only phospholipase Cγ was involved in GIT1-mediated VSMC apoptosis.

CONCLUSIONS

GIT1 is a novel mediator of vascular remodeling by regulating VSMC proliferation, migration, and apoptosis through phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling pathways.

Structures of dimeric GIT1 and trimeric beta-PIX and implications for GIT-PIX complex assembly

GIT (G protein-coupled receptor kinase-interacting protein) and PIX (p21-activated kinase-interacting exchange factor) family proteins integrate signaling pathways involving Arf and Rho family GTPases. GIT1 and beta-PIX form a constitutively associated complex that acts as a scaffold to allow the formation of large multiprotein assemblies that regulate synaptogenesis, cell polarity and cell migration among other physiological processes. Complex formation is mediated by the GIT binding domain (GBD) in beta-PIX, which recognizes the Spa homology domain of GIT1. Both binding domains are adjacent to predicted coiled-coil segments that allow homo-oligomerization of GIT1 and beta-PIX, respectively. Oligomerization of GIT and PIX proteins is important for their physiological functions, and deletion of the coiled-coil domains interferes with correct subcellular localization and the GEF (guanine nucleotide exchange factor) activity of PIX. We have solved the crystal structures of the CC domains of GIT1 and beta-PIX and determined the stoichiometry of complex formation between the two proteins in order to understand the molecular architecture of the GIT1-beta-PIX complex. The crystal structure of the CC domain of GIT1 solved at 1.4 A resolution shows a dimeric, parallel CC that spans 67 A in length. Unexpectedly, and in contrast to prevalent dimeric models, the structure of the CC region of beta-PIX determined at 2.8 A resolution, combined with hydrodynamic studies, reveals that this protein forms a parallel trimer. Furthermore, we demonstrate that dimeric GIT and trimeric PIX form an unusual high-affinity heteropentameric complex in which each Spa homology domain of the GIT1 dimer recognizes one GBD of the beta-PIX trimer, leaving one GBD unoccupied. These results can serve as a basis to better understand oligomerization-dependent GIT1-beta-PIX-regulated signaling events and provide an insight into the architecture of large signaling complexes involving GIT1 and beta-PIX.

The multifunctional GIT family of proteins

The G-protein-coupled receptor (GPCR)-kinase-interacting proteins 1 and 2 (GIT1 and GIT2) are ubiquitous multidomain proteins involved in diverse cellular processes. They traffic between three distinct cellular compartments (cytoplasmic complexes, focal adhesions and the cell periphery) through interactions with proteins including ARF, Rac1 and Cdc42 GTPases, p21-activated kinase (PAK), PAK-interacting exchange factor (PIX), the kinase MEK1, phospholipase Cgamma (PLCgamma) and paxillin. GITs and PIX cooperate to form large oligomeric complexes to which other proteins are transiently recruited. Activation of Rac1 and Cdc42 drives association of PAK with these oligomers, which unmasks the paxillin-binding site in GITs that recruits them to focal complexes. There, they regulate cytoskeletal dynamics by feedback inhibition of Rac1. GITs also participate in receptor internalization by regulating membrane trafficking between the plasma membrane and endosomes, targeting ARF GTPases through their ARF GTPase-activating protein (ARF-GAP) activity. Furthermore, GITs act as scaffolds to control spatial activation of several signaling molecules. Finally, recent results suggest pathogenic roles for GIT proteins in Huntington's disease and HIV infection.

ArfGAP1 responds to membrane curvature through the folding of a lipid packing sensor motif

ArfGAP1 promotes GTP hydrolysis in Arf1, a small G protein that interacts with lipid membranes and drives the assembly of the COPI coat in a GTP-dependent manner. The activity of ArfGAP1 increases with membrane curvature, suggesting a negative feedback loop in which COPI-induced membrane deformation determines the timing and location of GTP hydrolysis within a coated bud. Here we show that a central sequence of about 40 amino acids in ArfGAP1 acts as a lipid-packing sensor. This ALPS motif (ArfGAP1 Lipid Packing Sensor) is also found in the yeast homologue Gcs1p and is necessary for coupling ArfGAP1 activity with membrane curvature. The ALPS motif binds avidly to small liposomes and shows the same hypersensitivity on liposome radius as full-length ArfGAP1. Site-directed mutagenesis, limited proteolysis and circular dichroism experiments suggest that the ALPS motif, which is unstructured in solution, inserts bulky hydrophobic residues between loosely packed lipids and forms an amphipathic helix on highly curved membranes. This helix differs from classical amphipathic helices by the abundance of serine and threonine residues on its polar face.