of glutamines in the N-terminus of the protein. This region modulates the DNA binding activity of the C-terminus, and modulation of DNA-binding affects the rate of transcription complex formation and initiation of transcription. Mutations that expand the number of CAG repeats encoding this polyglutamine tract, and thus increase the length of the polyglutamine string, are associated with spinocerebellar ataxia 17, a neurodegenerative disorder classified as a polyglutamine disease. DNA-protein interactions When TBP binds to a TATA box within the DNA, it distorts the DNA by inserting amino acid side-chains between base pairs, partially unwinding the helix, and doubly kinking it. The distortion is accomplished through a great amount of surface contact between the protein and DNA. TBP binds with the negatively charged phosphates in the DNA backbone through positively charged lysine and arginine amino acid residues. The sharp bend in the DNA is produced through projection of four bulky phenylalanine residues into the minor groove. As the DNA bends, its contact with TBP increases, thus enhancing the DNA-protein interaction. The strain imposed on the DNA through this interaction initiates melting, or separation, of the strands. Because this region of DNA is rich in adenine and thymine residues, which base-pair through only two hydrogen bonds, the DNA strands are more easily separated. Separation of the two strands exposes the bases and allows RNA polymerase II to begin transcription of the gene. TBP's C-terminus composes of a helicoidal shape that (incompletely) complements the T-A-T-A region of DNA. This incompleteness allows DNA to be passively bent on binding. For information on the use of TBP in cells see: RNA polymerase I, RNA polymerase II, and RNA polymerase III. Protein–protein interactions TATA-binding protein has been shown to interact with: BRF1, BTAF1, C-Fos, C-jun, EDF1, GTF2B (TFIIB), GTF2A1 (TFIIA subunit 1), GTF2F1 (TFIIF subunit 1) GTF2H4 (TFIIH subunit 4), Mdm2, MSX1, NFYB, P53, PAX6, POLR2A, POU2F1, RELA, NR2B1, TAF1, TAF4, TAF5, TAF6, TAF7, TAF9. TAF10, TAF11, TAF13, and TAF15. Complex assembly The TATA-box binding protein (TBP) is required for the initiation of transcription by RNA polymerases I, II and III, from promoters with or without a TATA box. In the presence of a TATA-less promoter, TBP binds with the help of TBP-associated factors (TAFs). TBP associates with a host of factors, including the general transcription factors TFIIA, -B, -D, -E, and -H, to form huge multi-subunit pre-initiation complexes on the core promoter. Through its association