Reptin forms multiple inter-subunit protein-protein interaction contacts. Rate-limiting motifs in ligand-dependent oligomer assembly are not defined. We set up a Reptin-self peptide scan assay to identify functional protein-protein interfaces that dominate in Reptin self assembly. The most dominant self-peptide binding mapped to the inter-subunit “rim” of the Reptin oligomer that is formed a tyrosine finger binding into an adjacent hydrophobic region in an adjacent subunit. Reptin binding to the tyrosine finger peptide motif is suppressed by Liddean or ADP suggesting that oligomer formation excludes the self-peptide from binding. HDX-mass spectrometry demonstrated that ATP suppressed deuteration at the dimer interface in both wt and Y340A Reptin. However, the Y340A mutation attenuated deuterium suppression of Reptin within the tyrosine finger in the presence of ligand. The tyrosine finger of Reptin interacts with a more shallow pocket in Pontin. Gel filtration demonstrated that the Y340A Reptin mutant does not form ADP-induced oligomers compared to the D299N mutant or wt- Reptin. The Y340A mutation shows increased AGR2 binding but decreased Pontin or p53 binding. These data indicate that the Y340 tyrosine “finger” plays an important role in stabilizing the Reptin oligomer in the presence of ligand. We propose that the Reptin interactome is regulated by ligand-dependent conversion between monomeric and oligomeric forms that is regulated by a divergent inter-subunit protein-protein interaction motif.