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BTB/POZ Domain
Class:Ubiquitin processes

No Image The crystallized structure of the BTB domain from BCL6 forms a tightly intertwined butterfly-shaped homodimer that consists of a highly hydrophobic interface. The structure shown consists of a single BCL6 BTB domain/SMRT BBD peptide complex. The domain consists of a central cluster of α-helices flanked by short β-sheets at both ends of the molecule. The dimer contact is mediated by β1, α1, α2, β5 and α6 structures. A suggested ligand-binding site is found in a surface groove composed of a grouping of conserved residues within the BTB domain. The base of the groove consists of residues from the α1/β2 and β3/α2 loops and the cleft walls are formed by the α3/β4 loop. The formation of this groove is dependent on the dimerization of the BTB domain. 
Structure Reference:
Ahmad, K.F. et al. (2003) Molecular Cell 12(6), 1551-1564. "The BTB domain of BCL6 bound to SMRT (red)". PDB: 1R2B.

Domain binding and function:
The BTB domain is a protein-protein interaction module consisting of approximately 120 amino acids that is found in over 600 different proteins in organisms ranging from yeast to humans. The domain was first identified as a conserved sequence element in the developmentally regulated Drosophila proteins Broad-complex, Tramtrack and Bric-abrac. The BTB domain, also known as the POZ (poxvirus and zinc finger) domain, is often found at the N-termini of several zinc finger transcription factors as well as Shaw-type potassium channels. Experimental studies have strongly implicated the BTB domain in the regulation of gene expression through the local control of chromatin conformation. In several cases, the BTB domain has been shown to mediate protein oligomerization which subsequently prevents high affinity DNA binding. Both homotypic and heterotypic protein-protein interactions have been observed because the BTB domain can form dimers as well as mediating interactions with non-BTB domain containing proteins. An additional function of BTB proteins are to act as substrate-specific adaptors of Cul-3 ligases. Cul-3 ligases are part of the Ring-finger domain family of E3 ubiquitin ligases that form multiple-subunit complexes around a cullin scaffolding protein. In C.elegans, the BTB protein, MEL-26, forms a complex with Cul-3 and is required for the degradation of MEI-1, a microtubule-severing protein.
Examples of Proteins: 
BTB/POZ domain protein
Binding partner
MEL-26 MEI-1 via MATH (Mephrin and TRAF homology domain) domain of MEL-26
Zinc Finger Protein with interaction Domain (ZID) Homotypic interaction
Zinc Finger 5 Protein (ZF5) Homotypic interaction
BCL-6 Zinc Finger Transcription Repressor N-CoR corepressor, SMRT corepressor
GAG Zinc Finger Protein Tramtrack (T+k) via BTB heterotypic interaction

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