Crystal Structures of Human TBC1D1 and TBC1D4 (AS160) RabGTPase-activating Protein (RabGAP) Domains Reveal Critical Elements for GLUT4 Translocation

Abstract

We have solved the x-ray crystal structures of the RabGAP domains of human TBC1D1 and human TBC1D4 (AS160), at 2.2 and 3.5 Å resolution, respectively. Like the yeast Gyp1p RabGAP domain, whose structure was solved previously in complex with mouse Rab33B, the human TBC1D1 and TBC1D4 domains both have 16 ␣-helices and no ␤-sheet elements. We expected the yeast Gyp1p RabGAP/mouse Rab33B structure to predict the corresponding interfaces between cognate mammalian RabGAPs and Rabs, but found that residues were poorly conserved. We further tested the relevance of this model by Alascanning mutagenesis, but only one of five substitutions within the inferred binding site of the TBC1D1 RabGAP significantly perturbed catalytic efficiency. In addition, substitution of TBC1D1 residues with corresponding residues from Gyp1p did not enhance catalytic efficiency. We hypothesized that biologically relevant RabGAP/Rab partners utilize additional contacts not described in the yeast Gyp1p/mouse Rab33B structure, which we predicted using our two new human TBC1D1 and TBC1D4 structures. Ala substitution of TBC1D1 Met 930 , corresponding to a residue outside of the Gyp1p/Rab33B contact, substantially reduced catalytic activity. GLUT4 translocation assays confirmed the biological relevance of our findings. Substitutions with lowest RabGAP activity, including catalytically dead RK and Met 930 and Leu 1019 predicted to perturb Rab binding, confirmed that biological activity requires contacts between cognate RabGAPs and Rabs beyond those in the yeast Gyp1p RabGAP/mouse Rab33B structure.The trafficking of intracellular vesicles is regulated by Rab GTPases (Rab), which participate in Rab-mediated vesicle budding, uncoating, docking, and fusion (1, 2). Insulin-stimulated glucose uptake utilizes these processes during translocation of glucose transporter protein (e.g. GLUT4)3 vesicles from intracellular pools to the cell surface (3). TBC1D1 and TBC1D4 (also known as AS160) are Rab GTPase-activating proteins (RabGAPs) in skeletal myocytes and adipocytes, respectively, with functions in GLUT4 vesicle trafficking (4, 5). TBC1D1-and TBC1D4-catalyzed hydrolysis of Rab-bound GTP (active) to GDP (inactive) leaves GLUT4 vesicles sequestered in an intracellular compartment. Insulin-stimulated Akt phosphorylation of TBC1D1 and TBC1D4 decreases GTP hydrolysis, which increases Rab⅐GTP concentrations, GLUT4 vesicle translocation, and glucose uptake. Of Ͼ60 Rab proteins in mammalian genomes (1, 2), Ͻ20 associate with GLUT4 vesicles, and among them only Rabs 2A, 8A, 10, and 14 have been shown to be potential substrates for TBC1D1 or TBC1D4 (6 -8).TBC1D1 and TBC1D4 each contain ϳ1300 residues. Besides catalytic RabGAP domains at their carboxyl termini, each has two putative amino-terminal phosphotyrosine binding domains whose functions are under investigation. The second phosphotyrosine binding domain has been shown to bind insulin-regulated aminopeptidase, a marker for GLUT4 vesicle (9). TBC1D1 and TBC1D4 are closely related paralogs, with 47% overall identit...

The carboxy-terminal region of the TBC1D4 (AS160) RabGAP mediates protein homodimerization

“…The RabGAP itself eluted as a monomer (Fig. 2A), consistent with previous findings using analytical ultra-centrifugation and X-ray crystallographic analyses [11,12]. By contrast, RabGAP-CTR eluted as a mixture of mainly two molecular mass forms (Fig.…”

Affinity between TBC1D4 (AS160) phosphotyrosine-binding domain and insulin-regulated aminopeptidase cytoplasmic domain measured by isothermal titration calorimetry

“…The RabGAP catalytic function of TBC1D4 (and TBC1D1) is mediated by the carboxyl-terminal RabGAP domain ( Fig. 1), whose crystal structure has been determined (6).…”

“…The IRAP 51-109 was not expressed at levels high enough for the calorimetry experiment. The recombinant proteins were isolated on Nickel-NTA columns, and the His6-tag was removed with thrombin (Roche) cleavage as previously reported (6). The proteins were further purified using a Superdex 200 sizing column (GE Healthcare) in GF buffer (50 mM TRIS pH 7.5 and 150 mM NaCl), and concentrated by centrifugation (Amicon Centriprep).…”

TBC1D1 and TBC1D4 (AS160) RabGAP Domains are Characterized as Monomers in Solution by Analytical Ultracentrifugation

“…7 Since, TBC1D1 and TBC1D4 share 76% sequence identity over their RabGAP domains, the secondary structure elements mostly overlapped in the two structures which were predominately α-helical with no β-sheet elements. The 17 α-helices in TBC1D1 RabGAP domain were sequentially named from α1 to α16 ending with α16'.…”