Investigation of novel interaction partners of the transcription factor B-Myb and the role of B-Myb in the DNA-damage response

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Myb proteins constitute a family of transcription factors whose members (A-Myb, B-Myb and c- Myb) play key roles in regulating cell proliferation and differentiation. While the expression of AMyb and c-Myb is restricted to particular cell types, B-Myb is expressed in virtually all proliferating cells and is thought to play a general role during the cell cycle. The activity of B-Myb as a transcription factor is regulated by cyclinA/Cdk2-dependent phosphorylation as well as by interaction with other proteins and reaches a maximum during the S-phase of the cell cycle. Although several target genes for B-Myb have been identified the function of B-Myb is not yet well understood. Our recent work has provided insight into two novel aspects of B-Myb function. Knock-out studies have shown that B-Myb,, in addition to its role in the cell cycle, is also involved in the cellular response to DNA-damage. Furthermore, in an attempt to identify novel protein interaction partners of B-Myb we have found that B-Myb interacts with cullin3 and several BTBdomain proteins. CullinS is a scaffold protein of a class of ubiquitin ligase complexes and BTBproteins are thought to be substrate-specific adaptors that recruit substrate proteins to be ubiquitinated and subsequently degraded to the cullin3-based ubiquitin ligase. How these complexes act and how substrate proteins are selected is only beginning to be understood. The interaction of these proteins with B-Myb is therefore very interesting as it may provide further insight into the function of cullinS as well as the role of B-Myb in cell cycle progression and the DNA-damage response. The project described here addresses both novel aspects of B-Myb function. We plan to study the interaction of B-Myb, cullinS and BTB-proteins in detail and address several key issues raised by our findings. Is B-Myb itself a substrate of a cuIIinS-based ubiquitin ligase complex or is the ubiquitination of other proteins by this complex regulated by B-Myb? Is there a functional relevance to the interaction of B-Myb with certain BTB-proteins which is independent of cullinSmediated ubiquitination? Finally, what are the protein domains of B-Myb, cullin3 and the BTBproteins that interact with each other? Identification of these domains will provide the basis for the structural analysis of the interaction of these proteins. In addition to the protein interaction studies, we plan to characterize the signal transduction pathway that activates B-Myb in response to DNA-damage and identify downstream targets of B-Myb following DNA-damage. We think that the work described here will provide interesting novel insight into the role of B-Myb and the function of the cullinS-based ubiquitin ligase.