ZXDC

Abstract: The transcription of major histocompatibility complex class II (MHC II) genes is dependent on the co-activator protein class II trans-activator (CIITA). We have recently identified a protein known as zinc finger X-linked duplicated family member C (ZXDC) that, along with its binding partner ZXDA, forms a complex that interacts with CIITA and regulates MHC II transcription. Western blot analysis with anti-ZXDC antibodies identified two species of the ZXDC protein, one migrating near its predicted molecular mass and one with slower electrophoretic mobility. We report here that the slower migrating form is the result of sumoylation at a single lysine residue within the transcriptional activation domain of ZXDC. Three SUMO proteins (SUMO-1, -2 and -3) can modify the ZXDC protein. Multiple SUMO E3 ligase enzymes and HDAC4 can facilitate ZXDC sumoylation, and one ligase, PIASy, interacts with a specific region of the ZXDC protein. We found that sumoylation does not appear to disrupt or modulate the interaction of ZXDC with its binding partners. Rather, sumoylation of ZXDC is required for full activity of the transcriptional activation domain. Our findings suggest that sumoylation is an important regulator of ZXDC.

Abstract: The zinc finger X-linked duplicated A (ZXDA) and ZXDC proteins are both required for robust transcription of major histocompatibility complex class II (MHC II) genes. Aside from the full length ZXDC mRNA transcript, at least one additional mRNA is produced by the ZXDC gene, in which transcription initiates within the first exon and terminates within the seventh intron. The protein product produced from this transcript, which we have named ZXDC2, is truncated on both the N- and C-terminus. We demonstrate here that ZXDC2 functions to repress MHC II transcription induced in HeLa cells treated with IFN-γ. We further demonstrate that ZXDC2 interacts with both ZXDA and ZXDC, suggesting a mechanism by which ZXDC2 may inhibit MHC II transcription. These studies not only provide additional support for the role of ZXD proteins in regulating MHC II transcription, but also demonstrate a unique mechanism for the synthesis of a mRNA isoform.