Pulling a Ligase out of a "HAT": pCAF Mediates Ubiquitination of the Class II Transactivator.

Julie E Morgan,Susanna F Greer

doi:10.1155/2017/8093813

Julie E Morgan, Susanna F Greer

Open Access

https://doi.org/10.1155/2017/8093813

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Abstract

The Class II Transactivator (CIITA) is essential to the regulation of Major Histocompatibility Class II (MHC II) genes transcription. As the “master regulator” of MHC II transcription, CIITA regulation is imperative and requires various posttranslational modifications (PTMs) in order to facilitate its role. Previously we identified various ubiquitination events on CIITA. Monoubiquitination is important for CIITA transactivity, while K63 linked ubiquitination is involved in crosstalk with ERK1/2 phosphorylation, where together they mediate cellular movement from the cytoplasm to nuclear region. Further, CIITA is also modified by degradative K48 polyubiquitination. However, the E3 ligase responsible for these modifications was unknown. We show CIITA ubiquitination and transactivity are enhanced with the histone acetyltransferase (HAT), p300/CBP associated factor (pCAF), and the E3 ligase region within pCAF is necessary for both. Additionally, pCAF mediated ubiquitination is independent of pCAF's HAT domain, and acetylation deficient CIITA is K48 polyubiquitinated and degraded in the presence of pCAF. Lastly, we identify the histone acetyltransferase, pCAF, as the E3 ligase responsible for CIITA's ubiquitination.

Highlights

Major Histocompatibility Class II (MHC II) genes are essential for the initiation of adaptive immune responses to extracellular pathogens; their expression and activation are of critical importance and are tightly regulated [1,2,3]
We identify that Class II Transactivator (CIITA) mono, K63, and, K48 linked ubiquitination are mediated by p300/CBP associated factor (pCAF) in vivo. These results demonstrate pCAF’s capacity to facilitate various topologies of CIITA ubiquitination. These results indicate that pCAF, via its E3 ligase activity, plays additional important roles in the regulation of CIITA activity and in regulating the expression of MHC II genes
Once pCAF acetylates CIITA, CIITA accumulates in the nucleus, where it binds to the enhanceosome complex at the MHC II promoter [14] and drives MHC II transcription

Summary

Introduction

Major Histocompatibility Class II (MHC II) genes are essential for the initiation of adaptive immune responses to extracellular pathogens; their expression and activation are of critical importance and are tightly regulated [1,2,3]. PTMs on CIITA include phosphorylation, ubiquitination, and acetylation [11,12,13,14,15,16,17,18] These modifications precisely regulate CIITA’s location, function, and stability within the cell and increase CIITA activity at the MHC II promoter [8, 10, 13,14,15, 19,20,21,22]. PCAF’s intrinsic ubiquitination domain was identified and shown to play a role in the ubiquitination and stability of the critical cell cycle protein, human double minute 2 (the human ortholog of Mdm2) [23, 25], and Gli, a transcription factor that mediates hedgehog signaling [26]. As pCAF is known to affect the activity of many transcription factors

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