The objective of the research is to characterize the protein, MAX, a helix-loop-helix zipper protein that forms a sequence-specific DNA-binding complex with Myc, a protein in the protooncogene family. The myc protooncogene family has been implicated in cell proliferation, differentiation, and neoplasia, but its mechanism of function at the molecular level is unknown. The molecular structure of MAX is known, and the protein-complex has been crystallographically characterized. A cupric ion binding site has been identified, g = 2.26 and A = 167 G. Titration experiments indicate that there is a single cupric ion binding site per MAX homodimer. Part of the ligand set for this bound-copper ion is attributed to a pair of histidine residues at position 81 in the leucine zipper domain. The main focus of this summer research was to further characterize this Cu binding site and to determine the involvement of the His 81 residues in the homodimer form of MAX. In the purification of the 151 amino acid MAX, the protein is expressed with a 10 histidine leader peptide as well as a factor Xa cleavage site, which allows for the efficient isolation of the protein from a Ni-resin column using a gradient of EDTA. This preparation raises two concerns, one being an iron contamination seen in some of the early isolations, and secondly, is there an efficient removal of the poly His leader sequence? Most of the X-band EPR analyses were to check for iron contamination in the various preparations. In these analyses, I learned a new sample prepara tion forming an icicle and using a finger dewar as a sample holder. EPR spectra of all preparations of His-Max and of the mutant His81Leu-Max suggest that there was no iron contamination. The isotope 63Cu was added to form the MAX homodimer, again in a 1:1 ratio of metal to homodimer. These studies were run in a potassium phosphate buffer as opposed to the first titration experiments, which were run in sodium phosphate. Protein solutions in sodium phosphate were found to change pH significantly at low temperatures due to a decrease in the solubility of the monohydrogen species upon freezing. Further characterization of the His81Leu derivative was incomplete for the amino acid analysis showed that the mutant was not taken up in the desired position. Further work focused on preparing the His81Leu. Future studies will include low-frequency EPR analysis of the 63Cu-MAX homodimer to determine the number of nitrogen atoms bound to the metal center by analysis of the nitrogen superhyperfine lines. The His81Leu mutant will be analyzed to determine whether this position is significant in the binding of a metal ion.
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