Formation of transport vesicles involves polymerization of cytoplasmic coat proteins (COP). In COPI vesicle biogenesis, the heptameric complex coatomer is recruited to donor membranes by the interaction of multiple coatomer subunits with the budding machinery. Specific binding to the trunk domain of γ‐COP by the Golgi membrane protein p23 induces a conformational change that causes polymerization of the complex. Using single‐pair fluorescence resonance energy transfer, we find that this conformational change takes place in individual coatomer complexes, independent of each other, and that the conformational rearrangement induced in γ‐COP is transmitted within the complex to its α‐subunit. We suggest that capture of membrane protein machinery triggers cage formation in the COPI system.