The competitor-introduced Gγ recruitment system, a new approach for screening affinity-enhanced proteins

We have developed a new approach based on the G gamma recruitment system to screen affinity-enhanced proteins by expressing a binding competitor. The previously established G gamma recruitment system is a yeast two-hybrid (Y2H) system that utilizes G-protein signaling, and is based on the fact that membrane localization of the G-protein c subunit (G gamma) is essential for signal transduction in yeast. In the original Y2H system, an engineered G gamma that lacks membrane localization upon deletion of the lipid modi. cation site (G gamma(cyto)) is produced, and a candidate protein with an artificial lipidation site and its counterpart fused with G gamma(cyto) are expressed. As protein-protein interactions bring G gamma(cyto) towards the plasma membrane, G-protein signaling can be activated, and the interaction is detected by various cellular responses as the readout. In the current study, we expressed a third cytosolic protein that competes with the candidate protein to specifically isolate affinity-enhanced mutants from a mutation library of the candidate protein. Enhancing the affinity of the protein candidate guides the counterpart-G gamma(cyto) fusion protein towards the plasma membrane and activates signaling. Using mutants of the Z domain derived from Staphylococcus aureus protein A as candidate proteins or competitors, and the Fc portion of human immunoglobulin G (IgG) as the counterpart, we demonstrate that affinity-enhanced proteins can be effectively screened from a library containing a 10 000-fold excess of non-enhanced proteins. This new approach, called the competitor-introduced G gamma recruitment system, will be useful for efficient discovery of rare valuable candidates hidden among excess ordinary ones.