Human TAFII28 and TAFII18 interact through a histone fold encoded by atypical evolutionary conserved motifs also found in the SPT3 family

Determination of the crystal structure of the human Tap-associated factor (hTAF(II))28/hTAF(II)18 heterodimer shows that these TAF(II)s form a novel histone-like pair in the TFIID complex. The histone folds in hTAF(II)28 and hTAF(II)18 were not predicted from their primary sequence, indicating that these TAF(II)s define a novel family of atypical histone fold sequences. The TAF(II)18 and TAF(II)28 histone fold moths are also present in the N- and C-terminal regions of the SPT3 proteins, suggesting that the histone fold in SPT3 may be reconstituted by intramolecular rather than classical intermolecular interactions. The;existence of additional histone-like pairs in both the TFIID and SAGA complexes shows that the histone fold is a more commonly used motif for mediating TAF-TAF interactions than previously believed.