The Saccharomyces cerevisiae proteasome comprises a 19-subunit regulatory particle and a 28-subunit core particle. To be degraded, substrates must cross the core particle-regulatory particle interface, a site for complex conformational changes and regulatory events. This interface includes two aligned heteromeric rings, one formed by the six ATPase (Rpt) subunits of the regulatory particle and the other by the seven α subunits of the core particle. The Rpt C termini bind to intersubunit cavities in the α-ring, thus directing core particle gating and proteasome assembly. We mapped the Rpt C termini to the α subunit pockets, using a cross-linking approach that revealed an unexpected asymmetry: one side of the ring shows 1:1 contacts of Rpt2-α4, Rpt6-α3 and Rpt3-α2, whereas on the opposite side, the Rpt1, Rpt4 and Rpt5 tails each cross-link to multiple α pockets. Rpt-core particle cross-links are all sensitive to nucleotides, implying that ATP hydrolysis drives dynamic alterations at the core particle-regulatory particle interface.