The BTB domain (Broad-Complex, Tramtrack and Bric a brac) is also known as the POZ domain (POxvirus and Zinc finger). It is a homodimerization domain occurring at the N-terminus of proteins containing multiple copies of either zinc fingers of the C2H2 type (see <PDOC00028>) or Kelch repeats [1,2]. Many BTB proteins are transcriptional regulators that are thought to act through the control of chromatin structure.

The structure of the BTB domain of the promyelocytic leukemia zinc finger (PLZF) protein has been determined by X-ray crystallography and reveals a tightly intertwined dimer with an extensive hydrophobic interface [3]. A surface-exposed groove lined with conserved amino acids is formed at the dimer interface, suggesting a peptide-binding site.

Some proteins known to contain a BTB/TTK domain are listed below:

• Mammalian Bach proteins. These transcriptional regulators act as repressors or activators.
• Mammalian B-cell lymphoma 6 protein (BCL-6). A transcriptional regulator.
• Mammalian calicin. A possible morphogenic cytoskeletal element in spermiogenic differenciation.
• Human promyelocytic leukemia zinc finger (PLZF). A probable transcription factor.
• Drosophila GAGA transcription factor.
• Drosophila Mod(MDG4) or E(VAR)3-93D. A chromatin protein involved in gene silencing in position effect variegation, the control of gypsy insulator sequence, maintenance of gene expression, and apoptosis.
• Drosophila Tramtrack (TTK) protein. A probable transcriptional repressor.
• Drosophila Kelch protein. A component of ring canals that regulates the flow of cytoplasm between cells.
• Vaccinia virus proteins A55, C2, C4, C13, and F3.
• Myxoma virus proteins M-T8 and M-T9.