The class II trans-activator CIITA interacts with the TBP-associated factor TAFII32. 
The class II trans- activator (CIITA) is the main transcriptional co-activator for the expression of MHC class II proteins. Its N-terminal 125 amino acids function as an independent transcriptional activation domain. Analyses of the primary amino acid sequence of the activation domain predict the presence of three alpha-helices, each with a high proportion of acidic residues. Using site-directed mutagenesis, we found that two of these predicted alpha-helices are required for full transcriptional activation by CIITA. Moreover, a CIITA protein in which both functional alpha-helices have been deleted displays a dominant negative phenotype. This activation domain of CIITA interacts with the 32 kDa subunit of the general transcription complex TFIID, TAFII32. Decreased transcriptional activation by N-terminal deletions of CIITA is correlated directly with their reduced binding to TAFII32. We conclude that interactions between TAFII32 and CIITA are responsible for activation of class II genes.