Expression of MHC class I is dynamically regulated in response to a variety of stimuli. Agents such as TNF and interferon are well known inducers of class I transcription. In contrast, thyroid stimulating hormone (TSH) specifically reduces class I gene transcription in thyrocytes; this down-regulation is cAMP-mediated. Whereas previous studies in the laboratory have focused on the mechanisms of TSH-mediated repression, recent studies are examining the molecular basis of interferon-mediated induction through the transcriptional co-activator CIITA. The CIITA co-activator is essential for transcriptional activation of MHC class II genes and mediates enhanced MHC class I transcription. Class I promoter activation by CIITA requires both the downstream core promoter and upstream sequences. Activation is absolutely dependent on the upstream CRE, located between -100 and -107 bp, but is further enhanced by a series of upstream sequence elements. Interestingly, the DNA sequence requirements for CIITA mediated activation are distinct from those of constitutive transcription. Furthermore, the transcription factor requirements for CIITA activation are also distinct from those of constitutive transcription: constitutive transcription requires TAFII250 whereas CIITA activation does not. Of particular interest, we have found that CIITA contains an intrinsic acetyl transferase (AT) activity that maps to a region within the N-terminal segment of CIITA, between amino acids 36 and 132. This AT activity is regulated by the C-terminal GTP-binding domain. CIITA-mediated transactivation depends on the AT activity. Although predominantly localized in the nucleus, CIITA also is present in the cytoplasm and its subcellular distribution is actively regulated by the opposing actions of nuclear export and import. The nuclear export function is negatively regulated by the GTP-binding domain (GBD; aa 421-561) of CIITA: mutation or deletion of the GTP binding domain markedly increased export of CIITA from the nucleus. Remarkably, a CIITA GBD mutant binds CRM1/exportin significantly better than does wild type CIITA, leading to the conclusion that GTP is a negative regulator of CIITA nuclear export. In addition to the previously characterized N-and C-terminal nuclear localization signal (NLS) elements, there is an additional N-terminal nuclear localization activity, present between amino acids 209-222 which overlaps the proline, serine, threonine rich domain of CIITA. Thus, fine tuning of the nucleocytoplasmic distribution of co-activator proteins involved in transcription is an active and dynamic process that defines a novel mechanism for controlling gene regulation.
