HBO1 is a member of the human MYST family of acetyltransferases that is evolutionarily conserved from yeast to human. HBO1 functions in the context of a multi-protein histone acetyltransferase (HAT) complex containing JADE1/2/3, HBO1, ING4/5 and Eaf6 to regulate DNA replication, transcription, and other important cellular processes. HBO1 is shown to be responsible for the majority of H4 acetylation throughout the human genome. HBO1 functions in many important biological processes such as interaction with the origin recognition complex (ORC) and loading of the minichromosome maintenance protein complex (MCM), highlighting its importance in DNA replication. In addition, HBO1 is heavily co-localized to the transcription start site and H3K4me3 mark, which is one of the post translational modifications (PTMs) recognized by the PHD domains of another subunit in the HBO1 HAT complex containing HBO1, JADE, ING, Eaf. The activity of HBO1 is heavily dependent on the regulatory proteins within the HBO1 HAT complex. We demonstrate that the scaffold protein JADE functions as a platform to bring other subunits together forming a stable HBO1 HAT complex in addition to recruiting the substrate. Importantly, JADE is the main protein in the HBO1 HAT complex contributing to modulation of the activity and substrate specificity. Based on the fact that JADE contributes strongly to the overall activity of the HBO1 HAT complex, we dissected the molecular role of JADE in HBO1 histone acetyltransferase activity. We demonstrate that JADE contains 2 domains at the N-terminal end of the protein that cooperate to activate HBO1 activity towards H3/H4 substrate. Specifically, the Histone Core Biding Domain (HCBD) of JADE1 binds to the core of the histone and HBO1 and the Histone Binding Domain (HHBD) binds to both the HAT domain and the histone core, orienting the HAT domain towards the substrate. In addition, we demonstrated that JADE specifically binds to H3/H4 over H2A/H2B histones suggesting that the substrate recruitment is through specific interaction between JADE and H3/H4 rather than non-specific charge interaction. We demonstrate that HBO1 contains an N-terminal Histone Binding Domain (HBD) that makes additional contacts to H3/H4, independent of JADE interactions with histones. However, the additional HBO1 binding domain does not contribute to the overall HAT activity. Together, these studies highlight the importance JADE in HBO1 function.