Variable subunit contact and cooperativity of hemoglobins.

Tertiary structures of proteins are conserved better than their primary structures during evolution. Quaternary structures or subunit organizations, however, are not always conserved. A typical case is found in hemoglobin family. Although human, Scapharca, and Urechis have tetrameric hemoglobins, their subunit contacts are completely different from each other. We report here that only one or two amino acid replacements are enough to create a new contact between subunits. Such a small number of chance replacements is expected during the evolution of hemoglobins. This result explains why different modes of subunit interaction evolved in animal hemoglobins. In contrast, certain interactions between subunits are necessary for cooperative oxygen binding. Cooperative oxygen binding is observed often in dimeric and tetrameric hemoglobins. Conformational change of a subunit induced by the first oxygen binding to the heme group is transmitted through the subunit contacts and increases the affinity of the second oxygen. The tetrameric hemoglobins from humans and Scapharca have cooperativity in spite of their different modes of subunit contact, but the one from Urechis does not. The relationship between cooperativity and the mode of subunit contacts is not clear. We compared the atomic interactions at the subunit contact surface of cooperative and non-cooperative tetrameric hemoglobins. We show that heme-contact modules M3-M6 play a key role in the subunit contacts responsible for cooperativity. A module was defined as a contiguous peptide segment having compact conformation and its average length is about 15 amino acid residues. We show that the cooperative hemoglobins have interactins involving at least two pairs of modules among the four heme-contact modules at subunit contact.

Repetitive use of a phosphate-binding module in DNA polymerase beta, Oct-1 POU domain and phage repressors.

Motifs for sequence specific-protein-DNA interactions, such as helix-turn-helix, zinc finger and leucine zipper, are now better understood as a result of extensive studies of three-dimensional (3D) structures of transcription factors. On the other hand, little attention has been paid to motifs for sequence nonspecific binding, namely DNA-phosphate binding. To address the question whether different transcription factors and DNA manipulation enzymes, that is enzymes that work on DNA, share a similar mode of phosphate binding, we surveyed interactions between DNA and protein module, a structural unit of a globular protein. We analyzed the modular organization of DNA polymerase beta and found that residues making contact with DNA phosphates were localized to five modules. Structural comparison of these phosphate-binding modules against others in transcription factors and DNA manipulation enzymes revealed that DNA polymerase beta, the Oct-1 POU domain, 434 Cro and the Arc repressor have a phosphate-binding module with 3D structures similar to one another. This newly detected module, the phosphate-binding helix-turn-helix (pbHTH) module, named for its function and 3D structure, interacts with DNA by (i) making hydrogen bonds between a DNA phosphodiester oxygen and an amino hydrogen of the main chain located at the N-terminus of a C-terminal alpha-helix, and (ii) making electrostatic interactions between DNA phosphates and side chains of lysine or arginine. Finding structurally and functionally similar phosphate-binding units in different transcription factors and DNA manipulation enzymes suggests that shuffling of modules is not limited to the DNA base-recognition motif. Phosphate-binding modules are apparently also shuffled in DNA-binding proteins.