Regulatory subunit of the histone acetylase B (HAT-B) complex. The complex acetylates Lys-12 of histone H4 which is required for telomeric silencing. Component of the HAT-B complex composed of at least HAT1 and HAT2. The HAT-B complex binds to histone H4 tail. Belongs to the WD repeat RBAP46/RBAP48/MSI1 family.