CBP and p300 are histone acetyltransferases (HATs) that associate with and acetylate transcriptional regulators and chromatin. Mutations in their catalytic 'cores' are linked to genetic disorders, including cancer. Here we present the 2.8-Å crystal structure of the catalytic core of human p300 containing its bromodomain, CH2 region and HAT domain. The structure reveals that the CH2 region contains a discontinuous PHD domain interrupted by a RING domain. The bromodomain, PHD, RING and HAT domains adopt an assembled configuration with the RING domain positioned over the HAT substrate-binding pocket. Disease mutations that disrupt RING attachment led to upregulation of HAT activity, thus revealing an inhibitory role for this domain. The structure provides a starting point for understanding how chromatin-substrate targeting and HAT regulation are coupled and why mutations in the p300 core lead to dysregulation.
In a subset of poorly differentiated and highly aggressive carcinoma, a chromosomal translocation, t(15;19)(q13;p13), results in an in-frame fusion of the double bromodomain protein, BRD4, with a testis-specific protein of unknown function, NUT (nuclear protein in testis). In this study, we show that, after binding to acetylated chromatin through BRD4 bromodomains, the NUT moiety of the fusion protein strongly interacts with and recruits p300, stimulates its catalytic activity, initiating cycles of BRD4-NUT/p300 recruitment and creating transcriptionally inactive hyperacetylated chromatin domains. Using a patient-derived cell line, we show that p300 sequestration into the BRD4-NUT foci is the principal oncogenic mechanism leading to p53 inactivation. Knockdown of BRD4-NUT released p300 and restored p53-dependent regulatory mechanisms leading to cell differentiation and apoptosis. This study demonstrates how the off-context activity of a testis-specific factor could markedly alter vital cellular functions and significantly contribute to malignant cell transformation.
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