Abstract: Eaf1 and Eaf2 (Esa1-associated factor-1,2) have been identified as stable subunits of NuA4, a yeast histone H4/H2A acetyltransferase complex implicated in gene regulation and DNA repair. While both SANT-domain-containing proteins are required for normal cell cycle progression, their depletion does not affect global Esa1-dependent acetylation of histones. In contrast to all other subunits, Eaf1 is exclusively found associated with the NuA4 complex in vivo. It serves as a platform that coordinates the assembly of functional groups of subunits into the native NuA4 complex. Eaf1 shows structural similarities with human p400/Domino, a subunit of the NuA4-related TIP60 complex. On the other hand, p400 also possesses a SWI2/SNF2-family ATPase domain, which is absent from the yeast NuA4 complex. This domain is highly related to the yeast Swr1 protein, responsible for the incorporation of histone variant H2AZ in chromatin. Since all the components of the TIP60 complex are homologous to SWR1 or NuA4 subunits, we proposed that the human complex corresponds to physical merge of two yeast complexes. p400 function in TIP60 would then be accomplished in yeast by cooperation between SWR1 and NuA4. In agreement with such a model, NuA4 and SWR1 mutants show strong genetic interactions, NuA4 affects histone H2AZ incorporation/acetylation in vivo and both preset the PHO5 promoter for activation. Interestingly, expression of a chimeric Eaf1-Swr1 protein recreates a single human-like complex in yeast cells. Our results identified the key central subunit for the structure and functions of the NuA4 HAT complex and functionally linked this activity with the histone variant H2AZ from yeast to human cells.