Metal-responsive control of the expression of genes involved in metal metabolism and metal homeostasis allows an organism to tightly regulate the free or bioavailable concentration of beneficial metal ions, such as zinc, copper, and iron, within an acceptable range, while efficiently removing nonbeneficial or toxic metals. Emerging evidence also suggests that metal homeostasis is intimately coupled to the oxidative stress response in many cell types. The expression of genes that encode metallothioneins in all vertebrate cells is strongly induced by potentially toxic concentrations of zinc and cadmium, as well as in response to strong oxidizing agents, including hydrogen peroxide. This induction requires a cis-acting DNA element, termed a metal response element (MRE), and MRE-binding transcription factor-1 (MTF-1), a Cys2-His2 zinc finger protein. This review summarizes recent progress that has been made toward understanding the structure, function, and metalloregulation of mammalian MTF-1.

Retroviral nucleocapsid proteins (NCPs) are CCHC-type zinc finger proteins that mediate virion RNA binding activities associated with retrovirus assembly and genomic RNA encapsidation. Mason-Pfizer monkey virus (MPMV), a type D retrovirus, encodes a 96-amino acid nucleocapsid protein, which contains two Cys-X2-Cys-X4-His-X4-Cys (CCHC) zinc fingers connected by an unusually long 15-amino acid linker. Homonuclear, two-dimensional sensitivity-enhanced l5N-'H. three-dimensional "N-IH, and triple resonance NMR spectroscopy have been used to determine the solution structure and residue-specific backbone dynamics of the structured core domain of MPMV NCP containing residues 2 1-80. Structure calculations and spectral density mapping of N-H bond vector mobility reveal that MPMV NCP 2 1-80 is best described as two independently folded, rotationally uncorrelated globular domains connected by a seven-residue flexible linker consisting of residues 42-48. The N-terminal CCHC zinc finger domain (residues 24-37) appears to adopt a fold like that described previously for HIV-1 NCP; however, residues within this domain and the immediately adjacent linker region (residues 38-41) are characterized by extensive conformational averaging on the ps-ms time scale at 25 "C. In contrast to other NCPs, residues 49-77, which includes the C-terminal CCHC zinc-finger (residues 53-66). comprise a well-folded globular domain with the Val49-Pro-Gly-Leu52 sequence and C-terminal tail residues 67-77 characterized by amide proton exchange properties and "N RI, R2, and {'H-I'NN) NOE values indistinguishable to residues in the core C-terminal finger. Twelve refined structural models of MPMV NCP residues 49-80 (pairwise backbone RMSD of 0.77 A) reveal that the side chains of the conserved Pro50 and Trp62 are in van der Waals contact with one another. Residues 70-73 in the C-terminal tail adopt a reverse turn-like structure. Ile77 is involved in extensive van der Waals contact with the core finger domain, while the side chains of Ser68 and Am75 appear to form hydrogen bonds that stabilize the overall fold of this domain. These residues outside of the core finger structure are conserved in D-type and related retroviral NCPs, e.g., MMTV NCP, suggesting that the structure of MPMV NCP may be representative of this subclass of retroviral NCPs. Keywords:Mason-Pfizer monkey virus (MPMV) is the prototypical member of the primate D-type retroviruses that include simian AIDS retrovirus type 1 (SRV-I) and SRV-2, the causative agents of simian AIDS in captive macaque populations (Power et al., 1986). Mouse mammary tumor virus (MMTV) is a type B retrovirus most closely

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