Spectroscopic characterization of Co(II)-, Ni(II)-, and Cd(II)-substitutedwild-type and non-native retroviral-type zinc finger peptides

The nucleocapsid protein (NCP) from Mason-Pfizer monkey virus (MPMV) contai ns two evolutionary invariant Cys-X-2-Cys-X-4-His-X-4-Cys retroviral-type z inc finger structures, where the Cys and His residues provide ligands to a tetrahedrally coordinated Zn(II) ion. The N-terminal zinc finger (F1) of NC P from MPMV contains an immediately contiguous Cys in the -1 position relat ive to the start of this conserved motif: Cys-Cys-X-2-Cys-X-4-His-X-4-Cys. Metal complexes of 18-amino acid peptides which model the native zinc finge r sequence, SER-Cys-X-2-Cys-X-4-His-X-4-Cys (F1_SC), and non-native Cys-SER -X-2-Cys-X-4-His-X-4-Cys (F1_CS) and SER-SER-X-2-Cys-X-4-His-X-4-Cys (F1_SS ) sequences have been spectroscopically characterized and compared to the n ative two-zinc-finger protein fragment, MPMV NCP 21-80. All Co(II)-substitu ted peptide complexes adopt tetrahedral ligand geometries and have S-->Co(I I) ligand-to-metal charge-transfer (LMCT) transition intensities consistent with three Co(II)-S bonds for F1_SC and F1_CS. The non-native F1_CS peptid e binds Co(II) with K-Co = 1.5 x 10(6) M-1, comparable to that of the nativ e complex, and approximate to 100-fold tighter than F1_SS. Like the Co(II) derivative, the absorption spectrum of Ni(II)-substituted NCP 21-80 is most consistent with tetrahedral Ni(II) complexes with multiple thiolate donors . In contrast, Ni(II) complexes of F1_SC and F1_CS exhibit a single absorpt ion band in the 400-550 nm region (epsilon approximate to 200-300 M-1 cm(-1 )), distinct in the two complexes, assignable to a degenerate d-d transitio n envelope characteristic of non-native square-planar coordination geometry , and an intense LMCT transition in the UV (epsilon(255) approximate to 14, 000 M-1 cm(-1)). Cd(II) complexes have intense absorption in the UV (lambda (max)=233nm), with absolute intensities consistent with approximate to 5000 M-1 cm(-1) per Cd(II)-S bond. Cd-113 NMR spectroscopy of Cd-113 MPMV NCP g ives delta=649 ppm, consistent with S3N coordination. Co(II) and Cd(II) com plexes of non-native F1_CS peptides are more sensitive to oxidation by O-2, relative to F1_SC, suggestive of a higher lability in the non-native chela te. The implications of these findings for the evolutionary conservation of this motif are discussed.