Specificity of energy transfer to photosystem II by in vitro reassociated homologous and heterologous membrane-bound phycobilisomes

In a previous publication we have reported the in vitro reassociation of phycobiliproteins with thylakoids of Fremyella diplosiphon to form homologous, functional, membrane-bound phycobilisomes (Kirilovsky, D., Kessel, M. and Ohad, I (1983) Biochim. Biophys. Acta 724, 416–426). In the present work, using the same experimental system, we demonstrate the in vitro formation of heterologous, membrane-bound phycobilisomes. Analysis of phycobiliprotein association and binding curves disclosed two types of binding sites: specific sites which allow energy transfer to Photosystem II and non-specific sites which become occupied only after saturation of the Photosystem II specific sites. Binding to non-specific sites does not result in energy transfer. Both types of sites are present on cyanophyte thylakoids. Thylakoids of eukaryotic chloroplasts such as those of Chlamydomonas reinhardtii or Euglena gracilis can bind phycobiliproteins which reassociate to form intact membrane-bound phycobilisomes. However, only non-specific binding occurs in such heterologous systems. Limited proteolysis of membrane-bound phycobilisomes results in a rapid loss of the 94–95 kDa polypeptide assumed to be required for binding and energy transfer (Redlinger, T. and Gantt, E. (1982) Proc. Natl. Acad. Sci. USA 79, 5542–5546). Phycobilisomes lacking this polypeptide cannot bind to either specific or non-specific sites. Based on these results, we conclude that the 94–95 kDa polypeptide is required for the association of the phycobilisomes to both homologous and heterologous membranes; however, additional factors within the Photosystem II unit of cyanophytes are also required for establishing energy transfer.