Component of the SWR1 complex which mediates the ATP-dependent exchange of histone H2A for the H2A variant HZT1 leading to transcriptional regulation of selected genes by chromatin remodeling. Component of the NuA4 histone acetyltransferase complex which is involved in transcriptional activation of selected genes principally by acetylation of nucleosomal histones H4 and H2A. The NuA4 complex is also involved in DNA repair. Yaf9 may also be required for viability in conditions in which the structural integrity of the spindle is compromised. Component of the SWR1 chromatin-remodeling complex composed of at least ACT1, ARP4, RVB1, RVB2, ARP6, YAF9, VPS71, VPS72, SWC3, SWC4, SWC5, SWC7 and SWR1, and perhaps BDF1. Component of the NuA4 histone acetyltransferase complex composed of at least ACT1, ARP4, YAF9, VID21, SWC4, EAF3, EAF5, EAF6, EAF7, EPL1, ESA1, TRA1 and YNG2. Interacts with SWC4. The coiled-coil domain is required for assembly into the NuA4 complex. Present with 259 molecules/cell in log phase SD medium.