I have a known transmembrane protein expressed on the surface of an epithelial cell and I want to find an unknown bacterial surface protein that binds to it as a virulence factor. My initial thought was to simply perform co-immunoprecipitation, but I realised this would not be possible for two surface proteins. What are my options to identify the binding partner? Would I use membrane extracts? Would it be difficult to keep my membrane protein stable in vitro?