Phosphorylation and Ubiquitination of Degron Proximal Residues Are Essential for Class II Transactivator (CIITA) Transactivation and Major Histocompatibility Class II Expression

Major histocompatibility (MHC) class II molecules are cell surface glycoproteins that present extracellular antigens to CD4(+) T cells and are essential for initiation of the adaptive immune response. MHC class II expression requires recruitment of a master regulator, the class II transactivator (CIITA), to the MHC class II promoter. Post-translational modifications to CIITA play important roles in modulating CIITA mediated transcription of various genes in different cell types. We have previously linked regulation of CIITA to the Ubiquitin Proteasome System (UPS), and we and others have demonstrated that mono-ubiquitination of CIITA dramatically increases its transactivity whereas poly-ubiquitination leads to CIITA degradation. Here we identify three degron proximal lysine residues, Lys-315, Lys-330, and Lys-333, and a phosphorylation site, Ser-280, located within the CIITA degron, that regulate CIITA ubiquitination, stability, and MHC class II expression. Together, these findings contribute to the developing post-translational modification code for CIITA.