Metal-thiolate clusters in the C-terminal domain of human neuronal growth inhibitory factor (GIF)

Neuronal growth inhibitory factor (GIF), a metallothionein-like protein (metallothionein-3), impairs the survival and neurite formation of cultured neurons. Native GIF contains 4 Cu(I) and three Zn(ZI) ions organized in homometallic metal-thiolate clusters. However, the cluster localization is not known. In this study, the metal-thiolate clusters formed with monovalent and divalent metal ions in the C-terminal domain of human GIF [GIF(32-68)] containing 11 cysteines were investigated. The cluster formation was followed by using electronic absorption, circular dichroism (CD), and magnetic circular dichroism (MCD) spectroscopy, and in the case of Cu(I) complexes also by luminescence spectroscopy at 77 K. Spectroscopic studies on the Cu(I)-GIF(32-68) complexes showed the successive formation of two air-sensitive Cu4S8-9- and Cu6S11-clusters. With Zn(II) and Cd(II) ions, a well-defined M4S11-cluster is formed in which each metal ion is tetrahedrally coordinated by cysteine thiolates. In the Cd-113 NMR spectra of Cd-113(4)-GIF(32-68), recorded at 293 and 323 K, all four Cd-113 resonances at 672.8, 620.9, 629.6, and 564.2 ppm were observed only at 323 K. Their detection at elevated temperature indicates a conformational flexibility of this domain. Evidence for the existence of a Cds-GIF(32-68) complex, contaning two more weakly bound Cd(II) ions, was also obtained. The formation of this complex requires the transformation of some originally terminal thiolates of the Cd4S11-cluster to bridging thiolates, suggesting a more accessible cluster structure. Such properties of Cd-4-GIF(32-68) have not been observed with the Cd4S11-cluster in the isolated cr-domain (amino acids 31-61) of metallothioneins. The significance of Cu- and Zn-clusters for the structure of native GIF is discussed.