AROMATIC HISTIDINE INTERACTIONS IN THE ZINC FINGER MOTIF - STRUCTURAL INEQUIVALENCE OF PHENYLALANINE AND TYROSINE IN THE HYDROPHOBIC CORE

The classical Zn finger (Cys-X2,4-Cys-X3-Phe-X5-Leu-X2-His-X3,4-His). The phenycontains an aromatic-histidine interaction (underlined) in its hydrophobic core-whose importance is suggested by the marked destablization of a Phe --> Leu analogue [Mortishire-Smith, R. J., Lee, M. S., Bolinger, L., & Wright, P. E. (1992) FEBS Lett. 1, 11-15]. In some Zn finger sequences the central Phe is occasionally replaced by Tyr, and when present, this substitution is generally conserved among species. To investigate whether Tyr would participate in an analogous aromatic-histidine interaction, we have determined the solution structure in a Phe --> Tyr mutant domain. Its global fold (the betabetaalpha motif) is similar to that of the Phe domain. Although the variant Tyr maintains edge-to-face packing against the proximal histidine, the phenolic ring is displaced toward solvent. Such displacement increases the solvent accessibility of the Tyr p-OH group and reduces steric overlap (and possible electrostatic repulsion) between the Tyr O(zeta) and His pi electrons. The Tyr analogue exhibits reduced dynamic stability (as indicated by more rapid exchange of amide protons in D2O) and may alternate in rapid equilibrium between major and minor conformers. Inequivalent Tyr-His and Phe-His interactions are likely to be general features of Zn finger architecture. Molecular modeling based on the Zif268 cocrystal structure [Pavletich, N. P., & Pabo, C. O. (1991) Science 252, 809-817] suggests that the variant Tyr p-OH group may readily be positioned to contribute a novel hydrogen bond to a DNA phosphate.