An Src homology 3-like domain is responsible for dimerization of the repressor protein KorB encoded by the promiscuous IncP plasmid RP4.

KorB is a regulatory protein encoded by the conjugative plasmid RP4 and a member of the ParB family of bacterial partitioning proteins. The protein regulates the expression of plasmid genes whose products are involved in replication, transfer, and stable inheritance of RP4 by binding to palindromic 13-bp DNA sequences (5'-TTTAGC(G/C)GCTAAA-3') present 12 times in the 60-kb plasmid. Here we report the crystal structure of KorB-C, the C-terminal domain of KorB comprising residues 297-358. The structure of KorB-C was solved in two crystal forms. Quite unexpectedly, we find that KorB-C shows a fold closely resembling the Src homology 3 (SH3) domain, a fold well known from proteins involved in eukaryotic signal transduction. From the arrangement of molecules in the asymmetric unit, it is concluded that two molecules form a functionally relevant dimer. The detailed analysis of the dimer interface and a chemical cross-linking study suggest that the C-terminal domain is responsible for stabilizing the dimeric form of KorB in solution to facilitate binding to the palindromic operator sequence. The KorB-C crystal structure extends the range of protein-protein interactions known to be promoted by SH3 and SH3-like domains.