Rapid identification of key amino-acid-DNA contacts through combinatorial peptide synthesis.
BACKGROUND
Basic helix-loop-helix (bHLH) transcription factors are characterized by a conserved four-helix bundle that recognizes a specific hexanucleotide DNA sequence in the major groove. Previous studies have shown that amino acids in the basic region make base-specific contacts, whereas the HLH region is responsible for dimerization. Structural data suggest that portions of the loop region may be proximal to the DNA; however, the role of the loop in DNA-binding affinity and specificity has not been investigated.


RESULTS
Protein-DNA recognition by the Drosophila bHLH transcription factor Deadpan was probed using combinatorial solid-phase peptide synthesis methods. A series of bHLH peptide libraries that modulate amino acid content and length in the loop region was screened with DNA and peptide affinity columns, and analyzed using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). A functional bHLH peptide with reduced loop length was found, and Lys80 was unambiguously identified as the sole loop residue critical for DNA binding. Unnatural amino acids were substituted at this position to assess contributions of the terminal amino group and the alkyl chain length to DNA-binding affinity and specificity.


CONCLUSIONS
Using combinatorial solid-phase peptide synthesis methods and MALDI-MS, we were able to rapidly identify a key amino acid involved in DNA binding by a bHLH protein. Our approach provides a powerful alternative to current recombinant DNA methods to identify and probe the energetics of protein-DNA interactions.
Winston
R L
RL
Department of Molecular Biology, The Scripps Research Institute, University of California, La Jolla, Berkeley, CA 92037, USA.
Gottesfeld
J M
JM
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis*; D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins; D001665:Binding Sites; D002846:Chromatography, Affinity; D020650:Combinatorial Chemistry Techniques; D004268:DNA-Binding Proteins / Q000737:chemistry; D019281:Dimerization; D004330:Drosophila; D029721:Drosophila Proteins; D018257:Helix-Loop-Helix Motifs; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000737:chemistry; D019151:Peptide Library; D010455:Peptides / Q000138:chemical synthesis*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D014157:Transcription Factors / Q000737:chemistry*
D000596:Amino Acids; D051792:Basic Helix-Loop-Helix Proteins; D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D009687:Nuclear Proteins; D019151:Peptide Library; D010455:Peptides; D014157:Transcription Factors; C078020:Dpn protein, Drosophila