UNUSUAL SH3-LIKE DOMAIN IN PROTEIN KorB FROM RP4-INFECTED E. COLI
H. Delbrück1, G. Ziegelin2, E. Lanka2 & U. Heinemann1,3
1Forschungsgruppe Kristallographie,
Max-Delbrück-Centrum für Molekulare Medizin,
Robert-Rössle-Str. 10, D-13122 Berlin, Germany
2Max-Planck-Institut für Molekulare Genetik,
Ihnestr. 63, D-14195 Berlin, Germany
3Institut für Kristallographie, Freie
Universität, Takustr. 6, D-14195 Berlin, Germany
RP4 is a conjugative plasmid belonging to the Escherichia coli incompatibility group P. Expression of several RP4 genes is controlled by the product of the RP4 korB gene, an acidic 39 kD protein acting as a repressor of transcription [1]. KorB does not belong to any known family of bacterial transcriptional regulator proteins.
We have crystallized the C-terminal domain of KorB, KorB-C, in several forms and determined its three-dimensional structure at 1.7 A resolution using a SIRAS approach from crystals of space group P212121. The asymmetric unit of the crystal contains two dimers of KorB-C with non-crystallographic dyad symmetry each. The subunits are composed of five b-strands forming two antiparallel b-sheets crossing approximately at a right angle. This tertiary fold is typical for SH3 domains as present in several proteins involved in eukaryotic signal transduction. Whereas eukaryotic SH3 domains are known to bind to proline-rich protein domains [2], KorB-C dimerizes in a way not described before for SH3-like domains. Since both KorB and KorB-C form stable dimers in solution, it is concluded that KorB-C functions by stabilizing the dimeric form of KorB for binding to its symmetric 13bp DNA recognition sequence.