The transcription factor LAC9 from Kluyveromyces lactis-like GAL4 from Saccharomyces cerevisiae forms a Zn(II)₂Cys₆ binuclear cluster

The DNA binding domain of the transcription factor LAC9 contains 6 cysteine residues with spacing in the primary peptide sequence identical to that found in the DNA binding domain of the GAL4 transcription factor. In GAL4, the CysX₂CysX₆CysX₆CysX₂CysX ₆Cys motif has been shown to form a Zn(II)₂Cys₆ binuclear cluster (Pan, T. and Coleman, J. E. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 2077-2081), representing a new structure for a Zn(II)-containing transcription factor which differs from the "zinc finger" motif first described for TFIIIA. LAC9 has been shown to bind two Zn(II) ions (Halvorsen, Y. C., Nandabalan, K., and Dickson, R. D. (1990) J. Biol. Chem. 265, 13283-13289). The similarity of the amino acid sequence and the Cys spacing within the DNA binding domain suggest that LAC9 should also be capable of forming the Zn(II)₂Cys₆ cluster found in GAL4. A fragment of LAC9 consisting of 144 amino acid residues spanning the DNA binding domain has been prepared with ¹¹³Cd(II) substituted for the two native Zn(II) ions. ¹¹³Cd NMR of this fragment (denoted LAC9(85-228*)) has been carried out in an attempt to test the hypothesis that LAC9, like GAL4, forms a binuclear cluster. The chemical shifts of the two bound ¹¹³Cd(II) ions, 705 and 692 ppm respectively, are consistent with ligation of each ¹¹³Cd(II) ion to 4 sulfur atoms. The best model for such ligation is that two of the cysteine S^- form bridges between the two Cd(II) ions. Formation of a Zn(II)-Cd(II) hybrid form of LAC9(85-228*) has also been observed. We conclude that LAC9 contains a Zn(II)₂Cys₆ binuclear cluster as previously reported for GAL4.