This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. BTB domains are protein-protein interaction motifs can that form homomeric and heteromeric complexes. There are 43 BTB-Zinc Finger and 49 BTB-BACK-Kelch (BBK) proteins in humans (Stogios et al 2005). The BTB-ZF proteins are generally transcription factors, and the BBK proteins are generally adaptor proteins in cullin ubiquitin ligase E3 complexes. In both BTB-ZF and BBK proteins, the BTB domains self associate into higher oligomeric species. We are interested in studying how BTB domains affect the architectures of transcription factors and ubiquitin ligase complexes. 1) BTB-ZF proteins. We have solved a total of 9 crystal structures of BTB domains from BTB-ZF proteins with data collected at the APS (Ghetu et al 2008;Stogios et al 2007, Ahmad et al 2003, etc). These BTB domains are either homodimers or hexamers, implying that there are either two or six DNA binding motifs in the full length transcription factors. We plan to study a series of full-length proteins by SAXS in order to understand the topological arrangement of the DNA binding ZF domains within the full-length proteins, possibly in complex with DNA fragments. We have expressed nearly full-length PLZF and it is a monodisperse dimer by gel filtration and DLS. 2) BBK proteins. E3 ligase complexes are widely studied, but the issue of the oligomeric state of these complexes is poorly understood. BBK proteins oligomerize via their BTB domains, and we have in-house solution data showing both dimeric (2:2) and tetrameric (4:4) BBK/cullin complexes. We plan to collect SAXS data on a number of BBK/cullin complexes that have different stoicheometries in order to understand how BBK proteins orient their bound substrates in E3 complexes.