Component of the argonaute siRNA chaperone (ARC) complex which is required for histone H3K9 methylation, heterochromatin assembly and siRNA generation. The ARC complex contains mostly double-stranded siRNA. Inhibits the release of the siRNA passenger strand from ago1 together with arb2. Inhibits the slicer activity of ago1. Required for swi6 localization to the centromeric repeats. Component of the argonaute siRNA chaperone (ARC) complex composed of ago1, arb1 and arb2. Interacts with ago1. Predominantly nuclear.