Zn-alpha2-glycoprotein, an MHC class I-related glycoprotein regulator of adipose tissues: modification or abrogation of ligand binding by site-directed mutagenesis

Zn-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.

The effects of increased flow rates on linear dominance hierarchies and physiological function in brown trout, Salmo trutta

The formation of dominance hierarchies within groups of salmonid fish is well documented and stream tanks are often used to create environmentally relevant conditions in which to study this aspect of fish behaviour. Although stream tanks simulate the natural environment of the fish in many ways, they have limitations in that they provide the fish with a rather predictable and constant habitat. The present study illustrates that under these constant conditions, the behaviour of fish in their natural environment may not be truly represented. Under constant conditions hierarchies were formed among groups of four brown trout, Salmo trutta, and the dominant fish displayed physiological advantages. However, when an environmental perturbation of increased water flow, simulating a spate, was imposed, the social behaviour of the fish was altered and the physiological advantages of dominance were lost. Clearly, environmental changes affect the behaviour, and consequently the physiology, of salmonid fish, therefore the importance of taking environmental disturbances into consideration in studies of salmonid behaviour should not be underestimated.