The class II trans-activator (CIITA) is recognized as the master regulator of major histocompatibility complex (MHC) class II gene transcription and contributes to the transcription of MHC class I genes. To better understand the function of CIITA, we performed yeast two-hybrid with the C-terminal 807 amino acids of CIITA, and cloned a novel human cDNA named zinc finger, X-linked, duplicated family member C (ZXDC). The 858 amino acid ZXDC protein contains 10 zinc fingers and a transcriptional activation domain, and was found to interact with the region of CIITA containing leucine-rich repeats. Over-expression of ZXDC in human cell lines resulted in super-activation of MHC class I and class II promoters by CIITA. Conversely, silencing of ZXDC expression reduced the ability of CIITA to activate transcription of MHC class II genes. Given the specific interaction between the ZXDC and CIITA proteins, as well as the effect of ZXDC on MHC gene transcription, it appears that ZXDC is an important regulator of both MHC class I and class II transcription.

The transcription of major histocompatibility complex class II (MHC II) genes depends critically upon the activity of the class II trans-activator (CIITA) protein. We previously described a novel CIITA-binding protein named zinc finger X-linked duplicated family member C (ZXDC) that contributes to the activity of CIITA and the transcription of MHC II genes. Here, we examined the contribution of a closely related family member of ZXDC, the ZXDA protein, to MHC II gene transcription. ZXDA has a domain organization similar to ZXDC, containing ten zinc fingers and a transcriptional activation domain. Knockdown and overexpression of ZXDA demonstrated its importance in the transcriptional activation of MHC II genes. We found that ZXDA and ZXDC can self-associate, and also form a complex with each other. The regions of the two proteins that contain zinc fingers mediate these interactions. Importantly, we found that the ZXDA-ZXDC complex interacts with CIITA, rather than either protein alone. Given our additional finding that ZXDC is present at MHC II promoters in HeLa cells, prior to and after treatment with IFN-gamma, it appears that ZXDA and ZXDC form an important regulatory complex for MHC II gene transcription.