CBP (CREB-binding protein) and p300 (also known as CREBBP or KAT3A and EP300 or KAT3B, respectively) are two histone acetyltransferases (HATs) that associate with and acetylate transcriptional regulators and chromatin. The catalytic core of animal CBP-p300 contains a bromodomain, a CH2 region containing a discontinuous PHD domain interrupted by this RING domain, and a HAT domain. Bromodomain-RING-PHD forms a compact module in which the RING domain is juxtaposed with the HAT substrate-binding site. This RING domain contains only a single zinc ion-binding cluster instead of two; instead of a second zinc atom, a network of hydrophobic interactions stabilizes the domain. The RING domain has an inhibitory role. Disease mutations that disrupt RING attachment lead to upregulation of HAT activity. HAT regulation may require repositioning of the RING domain to facilitate access to an otherwise partially occluded HAT active site. Plant CBP-p300 type HATs lack a bromodomain whose role in the animal animal CBP-p300's is to bind acetylated histones; it has been suggested that these plant proteins may utilise a different domain or another bromodomain protein to perform this function [1]. This RING domain has also been referred to as DUF902.

1: Structure of the p300 catalytic core and implications for chromatin targeting and HAT regulation. Delvecchio M, Gaucher J, Aguilar-Gurrieri C, Ortega E, Panne D; Nat Struct Mol Biol. 2013;20:1040-1046. PMID:23934153