RUN-CBFbeta interaction in C. elegans: computational prediction and experimental verification.

The Runt domain proteins are eukaryotic transcription factors that regulate major developmental pathways. All members of this family contain a highly-conserved sequence-specific DNA binding domain: the Runt domain (RD). Structural and biochemical studies have shown that the Runt domain undergoes a conformational transition upon binding to DNA and that this process is regulated by an unrelated partner protein CBFbeta that enhances the DNA binding affinity of RD. Most of the reported studies on the Runt domain transcription factors were performed on proteins from mammals and Drosophila whereas very little has been known about the C. elegans RD protein, RUN, which provides the simplest model system for understanding the function of this class of transcription factors. We performed computational studies on RD domains from various species including C. elegans, Drosophila, and human, using the atom-atom contact surface area scoring method. The scoring analysis indicates that the DNA binding regulation of the C. elegans RD protein (CeRD) occurs via its interaction with a CBFbeta-like partner, as found for the human proteins, whereas a different mode of regulation may occur in the Drosophila system. Sequence, secondary structure and fold analyses of a putative CBFbeta protein identified in the C. elegans genome, CeCBFbeta, sharing a 22% identity with the human protein, predict a similar structure of this protein to that of the human CBFbeta protein. We produced the C. elegans proteins CeRD and CeCBFbeta in bacteria and confirmed their physical interaction as well as cross interactions with the corresponding human proteins. We also confirmed the structural similarity of CBFbeta and CeCBFbeta by circular dichroism analysis. The combined results suggest that a similar mechanism of regulation operates for the human and the C. elegans RD proteins despite the low sequence identity between their CBFbeta proteins and the evolutionary distance between the two systems.