Browsing Biomedical and biological science by Title
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ZAG and a potential role in systemic lipid homeostastis: examining the evidence from in vitro human studies and patients with chronic illnessZAG, a 42 kDa ubiquitously expressed soluble and secreted protein, consists of an MHC-like fold and binds fatty acids with an affinity similar to that of albumin. In human adipocytes, cytoplasmic and secreted ZAG appear inversely related to fat mass. Immunoblotting and biochemical measurements of human cachectic adipocytes and plasma suggest that ZAG’s lipolytic function may be mediated by βIAR. Plasma ZAG correlates with cholesterol in human populations, as does its single nucleotide polymorphism rs4215, which also associates with obesity. Biochemical and human genetic data, in vitro experiments and theoretical data imply that adipocyte ZAG’s expression is regulated by PPAR-γ and glucocorticoids. Cell biological experiments and data from human tissue indicate that ZAG may induce fatty-acid oxidation in skeletal muscle. Overall, these findings suggest ZAG’s participation in systemic lipid homeostasis. Understanding the molecular mechanisms behind ZAG’s in vivo behavior potentially allows for rational drug design to control body fat mass.
Zinc-induced oligomerisation of zinc α2 glycoprotein reveals multiple fatty acid binding sitesZinc α2 glycoprotein (ZAG) is an adipokine with a class I major histocompatibility complex protein fold and is associated with obesity and diabetes. Although its intrinsic ligand remains unknown, ZAG binds the dansylated C11 fatty acid, DAUDA, in the groove between the α1 and α2 domains. The surface of ZAG has about 15 weak zinc binding sites deemed responsible for precipitation from human plasma. Here the functional significance of these metal sites was investigated. Analytical ultracentrifugation and circular dichroism showed that zinc, but not other divalent metals, cause ZAG to oligomerise in solution. Thus ZAG dimers and trimers were observed in the presence of 1 mM and 2 mM zinc. Molecular modelling of X-ray scattering curves and sedimentation coefficients indicated a progressive stacking of ZAG monomers, suggesting the ZAG groove may be occluded in these. Using fluorescence-detected sedimentation velocity, these ZAG-zinc oligomers were again observed in the presence of the fluorescent boron dipyrromethene fatty acid C16-BODIPY. Fluorescence spectroscopy confirmed that ZAG binds C16-BODIPY. ZAG binding to C16-BODIPY, but not to DAUDA, was reduced by increased zinc concentrations. We conclude that the lipid binding groove in ZAG contains at least two distinct fatty acid binding sites for DAUDA and C16-BODIPY, similar to the multiple lipid binding seen in the structurally-related immune protein Cd1c. In addition, because high concentrations of zinc occur in the pancreas, the perturbation of these multiple lipid binding sites by zinc may be significant in Type 2 diabetes where dysregulation of ZAG and zinc homeostasis occurs.
Zn-alpha2-glycoprotein, an MHC class I-related glycoprotein regulator of adipose tissues: modification or abrogation of ligand binding by site-directed mutagenesisZn-alpha(2)-glycoprotein (ZAG) is a soluble lipid-mobilizing factor associated with cancer cachexia and is a novel adipokine. Its X-ray crystal structure reveals a poly(ethylene glycol) molecule, presumably substituting for a higher affinity natural ligand, occupying an apolar groove between its alpha(1) and alpha(2) domain helices that corresponds to the peptide binding groove in class I MHC proteins. We previously provided evidence that the groove is a binding site for hydrophobic ligands that may relate to the protein's signaling function and that the natural ligands are probably (polyunsaturated) fatty acid-like. Using fluorescence-based binding assays and site-directed mutagenesis, we now demonstrate formally that the groove is indeed the binding site for hydrophobic ligands. We also identify amino acid positions that are involved in ligand binding and those that control the shape and exposure to solvent of the binding site itself. Some of the mutants showed minimal effects on their binding potential, one showed enhanced binding, and several were completely nonbinding. Particularly notable is Arg-73, which projects into one end of the binding groove and is the sole charged amino acid adjacent to the ligand. Replacing this amino acid with alanine abolished ligand binding and closed the groove to solvent. Arg-73 may therefore have an unexpected dual role in binding site access and anchor for an amphiphilic ligand. These data add weight to the distinctiveness of ZAG among MHC class I-like proteins in addition to providing defined binding-altered mutants for cellular signaling studies and potential medical applications.