Practice question

Co-immunoprecipitation evaluates tissue specific protein-protein interactions. Lysates from different tissues are precipitated with antibody against bait protein Apt1 immobilized on beads, co-precipitating interacting partners under native conditions. Precipitates are resolved by SDS-PAGE and probed for prey Bpt5. If Apt1 pull-down succeeds in liver lysate but Bpt5 signal is absent while other tissues show robust Bpt5 co-precipitation, interpretation is that Bpt5 protein is not expressed in liver, therefore unavailable for complex formation. This explains absence as expression difference, not as universal promiscuity or repressor activity.