THE CRYSTAL-STRUCTURE OF THE IMMUNITY PROTEIN OF COLICIN E7 SUGGESTS A POSSIBLE COLICIN-INTERACTING SURFACE

The immunity protein of colicin E7 (ImmE7) can bind specifically to th e DNase-type colicin E7 and inhibit its bactericidal activity, Here we report the 1.8-Angstrom crystal structure of the ImmE7 protein. This is the first x-ray structure determined in the superfamily of colicin immunity proteins. The ImmE7 protein consists of four antiparallel alp ha-helices, folded in a topology similar to the architecture of a four -helix bundle structure, A region rich in acidic residues is identifie d, This negatively charged area has the greatest variability within th e family of DNase-type immunity proteins; thus, it seems likely that t his area is involved in specific binding to colicin. Based on structur al, genetic, and kinetic data, we suggest that all the DNase-type immu nity proteins, as well as colicins, share a ''homologous-structural fr amework'' and that specific interaction between a colicin and its cogn ate immunity protein relies upon how well these two proteins' charged residues match on the interaction surface, thus leading to specific im munity of the colicin.