This study characterized how three adaptor proteins — pICln, RioK1 and COPR5 — bind the PRMT5–MEP50 methyltransferase complex, mapping a shared consensus binding motif and resolving the interactions structurally to inform inhibitor design in cancer biology. Flow Induced Dispersion Analysis (FIDA) was used to measure the dissociation constants (KD) of the adaptor peptides binding PRMT5–MEP50 in solution, yielding low-nanomolar affinities and providing an orthogonal cross-check to fluorescence polarization. It is an example of using FIDA to measure protein–protein binding affinity in solution.