Identification of protein-protein contacts between α/β-type small, acid-soluble spore proteins of Bacillus species bound to DNA

Abstract

Small, acid-soluble spore proteins (SASP) of the α/β-type from several Bacillus species were cross-linked into homodimers, heterodimers and homooligomers with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) in the presence of linear plasmid DNA. Significant protein cross-linking was not detected in the absence of DNA. In all four α/β-type SASP examined, the amino donor in the EDC induced amide cross-links was the α-amino group of the protein. However, the carboxylate containing amino acid residues involved in cross-linking varied. In SASP-A and SASP-C of Bacillus megaterium two conserved glutamate residues, which form part of the germination protease recognition sequence, were involved in cross-link formation. In SspC from Bacillus subtilis and Bce1 from Bacillus cereus the acidic residues involved in cross-link formation were not in the protease recognition sequence, but at a site closer to the N terminus of the proteins. These data indicate that, although there are likely to be subtle structural differences between different α/β-type SASP, the N-terminal regions of these proteins are involved in protein-protein interactions while in the DNA bound state.