title: Heterodimerization of the yeast homeodomain transcriptional regulators alpha 2 and a1 induces an interfacial helix in alpha 2.
abstract: The homeodomain proteins a1 and alpha 2 act cooperatively to regulate cell type specific genes in yeast. The basis of the cooperativity is a weak interaction between the two proteins which forms heterodimers that bind DNA tightly and specifically. In this paper, we examine the mechanism of heterodimerization. We show that two relatively small fragments of a1 and alpha 2 are capable of heterodimerization and tight DNA binding. The alpha 2 fragment contains the homeodomain followed by the natural 22 C-terminal amino acids of the protein; these 22 amino acids are unstructured in the alpha 2 fragment. The a1 fragment contains only the homeodomain, indicating that the a1 homeodomain mediates both DNA binding and protein-protein interactions with alpha 2. We used isotope-edited NMR spectroscopy to study the interaction in solution of these two fragments. Samples in which only the alpha 2 fragment was uniformly labeled with 15N allowed us to visualize changes in the NMR spectra of the alpha 2 fragment produced by heterodimerization. We found that the a1 homeodomain perturbs the resonances of only the C-terminal tail of alpha 2; moreover, contact with a1 converts a portion of this tail (residues 193-203) from its unstructured state to an alpha-helix, as determined by J coupling and NOE measurements. Thus the heterodimerization of two homeodomain proteins involves the specific interaction between a tail of one protein and the homeodomain of the other. This interaction is accompanied by the acquisition of secondary structure in the tail.
authors.affiliation: Institute of Molecular Biology, University of Oregon, Eugene 97403.
authors.forename: C L
authors.initials: CL
authors.lastname: Phillips
authors.forename: M R
authors.initials: MR
authors.lastname: Stark
authors.forename: A D
authors.initials: AD
authors.lastname: Johnson
authors.forename: F W
authors.initials: FW
authors.lastname: Dahlquist
mesh_terms: D000595:Amino Acid Sequence; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D018398:Homeodomain Proteins; D007700:Kinetics; D046911:Macromolecular Substances; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D015778:Minor Histocompatibility Antigens; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides / Q000737:chemistry / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry / Q000302:isolation & purification; D017433:Protein Structure, Secondary; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D051818:Replication Protein C; D012097:Repressor Proteins; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D014158:Transcription, Genetic
chemical_list: C096816:BCL2-related protein A1; D004268:DNA-Binding Proteins; D005656:Fungal Proteins; D018398:Homeodomain Proteins; C096063:MATA1 protein, S cerevisiae; C062928:MATA2 protein, S cerevisiae; D046911:Macromolecular Substances; D015778:Minor Histocompatibility Antigens; D009838:Oligodeoxyribonucleotides; D010446:Peptide Fragments; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D051818:Replication Protein C