Sp1 Zinc Finger Research Articles

Nonmyogenic Factors Bind Nicotinic Acetylcholine Receptor Promoter Elements Required for Response to Denervation

Nicotinic acetylcholine receptors (AChRs) belong to a class of muscle proteins whose expression is regulated by muscle electrical activity. In innervated muscle fiber, AChR genes are transcriptionally repressed outside of the synapse, while after denervation they become reexpressed throughout the fiber. The myogenic determination factors (MDFs) of the MyoD family have been shown to play a central role in this innervation-dependent regulation. In the chicken AChR alpha-subunit gene promoter, two E-boxes that bind MDFs are necessary to achieve the enhancement of transcription following muscle denervation. However, the deletion of promoter sequences located upstream to these E-boxes greatly impairs the response to denervation (Bessereau, J. L., Stratford- Perricaudet, L. D., Piette, J., Le Poupon, C. and Changeux, J. P. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 1304-1308). Here we identified two additional cis-regulatory elements of the alpha-subunit gene promoter that cooperate with the E-boxes in the denervation response. One region binds the Sp1 and Sp3 zinc finger transcription factors. The second region binds at least three distinct factors, among which we identified an upstream stimulatory factor, a b-ZIP-HLH transcription factor. We propose that among MDF-responsive muscle promoters, a specific combination between myogenic and nonmyogenic factors specify innervation-dependent versus innervation-independent promoters.

Unique DNA binding mode of the N-terminal zinc finger of transcription factor Sp1.

Transcription factor Sp1 has three tandem repeats of a Cys2His2-type zinc finger motif and specifically binds to GC box DNA. Although the DNA binding mode of the three zinc fingers of Sp1 is predicted to be similar to that of Zif268, this model does not explain the DNA binding property of the N-terminal zinc finger (finger 1). To understand the DNA recognition mode of Sp1, we have performed detailed analyses for the contribution of finger 1 to the high-affinity binding to the GC box DNA and for the interaction mechanism between finger 1 and DNA. Results of electrophoretic analyses using finger-deleted mutants of Sp1 and GC box mutants in the finger-contacting subsite demonstrate that the contribution of finger 1 to the total DNA binding affinity is lower than that of the C-terminal finger 3 but is dispensable for the high-affinity binding. The DNA sequence selectivity of finger 1 at the 3'-portion of the GC box is lower than that of fingers 2 and 3 at the 5'-portion. Alanine scanning mutagenesis in the alpha-helix of finger 1 reveals that Lys-1 immediately preceding the helix is important for the recognition of the two guanine bases, but other putative key amino acids do not affect the DNA binding. These results demonstrate that (1) the contribution of finger 1 to the DNA binding affinity and the sequence selectivity of Sp1 is smaller than that of fingers 2 and 3 and (2) the interaction mechanism between finger 1 and DNA is different from the Zif268 model. DNA interaction of Sp1 finger 1 has also been discussed in connection with that of TFIIIA or WT1.

Properties of the Sp1 zinc finger 3 peptide: coordination chemistry, redox reactions, and metal binding competition with metallothionein.

Toxic and/or carcinogenic consequences may result from metal ion substitution for the Zn-(II) in transcription factors containing zinc fingers, and the small Cys-rich metal-binding protein metallothionein (MT) may play a role in this metal substitution. To begin to evaluate this hypothesis, with regard to the carcinogenic metal ion Ni(II), a peptide corresponding to the third finger of the transcription factor Sp1 (Sp1-3) has been synthesized and its metal binding and redox reactions have been studied. The peptide binds Zn(II), Co(II), and Ni(II), with spectroscopic data indicating a tetrahedral coordination for the latter two; metal ion affinities have been quantified (Kd = 6 (+/- 3) x 10(-10), 3 (+/- 1) x 10(-7), and 4 (+/- 1) x 10(-6), respectively) and found to be less than those of an optimized zinc finger peptide (Krizek, B. A., Merkle, D. L., and Berg, J. M. (1993) Inorg. Chem. 32, 937-940) but greater than those of the second finger of transcription factor IIIA (Berg, J. M., and Merkle, D. L. (1989) J. Am. Chem. Soc. 111, 3759-3761). Reactions of the peptide and its metal-bound forms with dioxygen or hydrogen peroxide did not produce oxygen radical species; however, oxidation of the two Sp1-3 cysteines was modulated by metal ions (Zn < Co = apo < Ni), suggesting a protective role for Zn(II) but an enhancing role for Ni(II). Metal binding competition between Sp1-3 and the alpha domain of human liver MT-2 (alpha-hMT2) indicates a similar affinity for Zn(II). However, alpha-hMT2 has a higher affinity for Ni(II), suggesting that MT may play a protective role by ensuring Zn(II), rather than Ni(II), coordination to zinc finger sequences of transcription factors.

Nucleosome binding by the constitutive transcription factor Sp1.

The constitutive transcription factor Sp1 plays a role in the transcription of numerous viral and cellular genes including constitutive "housekeeping" genes and inducible genes. Sp1 has also been implicated in the formation of a nucleosome-free region in Simian virus 40 (SV40) minichromosomes. To investigate the potential functions of Sp1 in remodeling chromatin structures, Sp1 was analyzed for the ability to bind its recognition sites (GC boxes) in DNA fragments reconstituted into nucleosome cores. Sp1 was found to bind to the GC boxes of the SV40 early promoter when this element was reconstituted into nucleosome cores. The affinity of Sp1 for the nucleosomal SV40 early promoter DNA was reduced approximately 10-20-fold relative to naked DNA. A peptide containing only the zinc fingers of Sp1 was also capable of binding nucleosomal DNA, indicating that the glutamine-rich and serine/threonine-rich domains of Sp1 are not required for nucleosome binding. The binding of Sp1 to nucleosome cores resulted in the formation of a ternary Sp1-nucleosome complex.

Alteration of DNA Binding Specificity by Nickel (II) Substitution in Three Zinc (II) Fingers of Transcription Factor Sp1

Transcription factor Sp1, which has a DNA binding domain composed of three zinc fingers, binds to GC box (consensus sequence, G/T-GGGCGG-G/A-G/A-C/T) and activates the transcription by RNA polymerase II. Metal substitution of nickel(II) for zinc(II) in Sp1 causes no differences in the mode of protein-DNA interaction. However, sequence preference of Ni(II)Sp1 changes from 5′-GGGGCGGGGC to 5′-GGGGCGTGGC, and is distinct from that of Zn(II)Sp1. The result indicates an important effect of metal-induced folding on sequence-specific recognition of DNA by zinc-finger proteins.

Transition metals modulate DNA-protein interactions of SP1 zinc finger domains with its cognate target site

The metal free apoprotein of recombinant human transcription factor SP1 was used in metal reconstitution experiments to study the importance of zinc in facilitating DNA binding of zinc finger proteins. Our functional analysis indicates that several transition metals are capable of modulating DNA-protein interactions of zinc finger domains with their cognate DNA target sites. Excess or deficiency of divalent zinc, or the presence of transition metals, such as divalent cadmium, cobalt, copper, manganese and nickel impair DNA binding of zinc reconstituted SP1. In addition, functionally active SP1 protein can be obtained by metal reconstitutions in absence of zinc(II) by presence of cadmium(II)- and cobalt(II)-, to lesser extents by presence of nickel(II)- or manganese(II)chloride. This study indicates that zinc might play a functional role in regulating DNA protein interactions of zinc finger proteins in vivo. It is postulated that fluctuating divalent zinc alone or transition metals bound to cellular components might form mixed-ligand complexes that alter the zinc finger protein conformation and impair DNA binding.

Amino acid substitutions in the SP1 zinc finger domain alter the DNA binding affinity to cognate SP1 target site

Non conserved amino acids, which are located in the postulated alpha-helical region of the third zinc finger in human transcription factor SP1, have been replaced by amino acids, which occur at the analogous zinc finger position in human protein Kox 15. This helical domain was mutated from SDHLSKH to SSHLIOH (SP1-M3). Aspartic acid (D), serine (S) and lysine (K) were substituted by serine (S), isoleucine (I) and glutamine (Q). The DNA binding of the mutated SP1-M3 protein to the SP1 cognate target site GGG GCG GGG was significantly impaired, indicating that the amino acids, aspartic acid, serine and lysine play a pivotal role in DNA recognition. The mutated SP1 finger cannot imitate the function of the wild type SP1 finger in interacting with the cognate SP1 target site. This structure-function analysis indicates that the third SP1 zinc finger participates in sequence-specific DNA recognition. Thus, the affinity of zinc finger domains can be altered by substituting a limited number of amino acids. This observation is consistent with the notion that zinc finger domains are positioned in the major groove of the DNA and wrap around the DNA. Structure-function analysis of this kind might lead to the description of a zinc finger specific recognition code.

Base sequence discrimination by zinc-finger DNA-binding domains

Zinc fingers constitute important eukaryotic DNA-binding domains, being present in many transcription factors. The Cys2/His2 zinc-finger class has conserved motifs of 28-30 amino acids which are usually present as tandem repeats. The structure of a Cys2/His2 zinc finger has been determined by nuclear magnetic resonance, but details of its interaction with DNA were not established. Here we identify amino acids governing DNA-binding specificity using in vitro directed mutagenesis guided by similarities between the zinc fingers of transcription factors Sp1 and Krox-20. Krox-20 is a serum-inducible transcription activator which is possibly involved in the regulation of hindbrain development; it contains three zinc fingers similar to those of Sp1 and binds to a 9-base-pair target sequence which is related to that of Sp1. Our results show that each finger spans three nucleotides and indicate two positions in Krox-20 zinc fingers that are important for base-pair selectivity. Modelling with molecular graphics suggests that these residues could bind directly with the bases and that other amino acid-base contacts are also possible.