| The BTB domain is a highly conserved, widely distributed protein-protein interaction motif commonly found at the extreme N-terminus of zinc-finger transcription factors. Encompassing approximately 120 residues, this domain has been shown to mediate homo-oligomerization, hetero-oligomerization, and interactions with non-BTB domain proteins. Several BTB/zinc-finger proteins, such as B-cell lymphoma 6 (BCL-6) and the promyelocytic leukemia zinc finger protein (PLZF) are implicated in human malignancy. The biological properties of BTB-containing proteins are largely a result of the protein-protein interactions made by the domain. For example, the BTB domains from PLZF and BCL-6 function as autonomous transcriptional repression domains through their interactions with components of a histone deacetylase complex.; This thesis presents structural and biochemical studies on the PLZF and BCL-6 BTB domains. Such work was used to establish the structural basis for the specificity and stability of intermolecular BTB interactions, provide a framework for a mutational analysis to provide a structure-function model for understanding the dimerization and transcriptional repression of BTB domain containing proteins, and identify sites of interaction with other proteins.; Chapter 2 describes crystal structure of the PLZF BTB domain. The structure reveals a tightly intertwined dimer with an extensive hydrophobic interface, typical of obligate homodimers. In addition, a surface exposed groove lined with conserved amino acids is formed at the dimer interface, which was thought to represent the site of interaction of the PLZF BTB domain with co-repressors.; Chapter 3 presents a structure-based mutational analysis of the PLZF BTB domain, in conjunction with Jonathan Licht and Ari Melnick, Mount Sinai School of Medicine, New York City. This analysis identified a number of residues within the PLZF BTB domain critical for proper folding and transcriptional repression.; Chapter 4 presents the identification of a 17-residue fragment of the transcriptional co-repressor SMRT that binds to the BCL-6 BTB domain, and determination of the co-crystal structure. Two SMRT fragments bind symmetrically to the BCL-6 BTB homodimer. In combination with biochemical and in vivo data, the structure provides insight into the basis of transcriptional repression by BCL-6. |
