Chromophore Attachment in the Cyanobacterial Light Harvesting Proteins

Light harvesting in the cyanobacteria is conducted by a complex, self-assembling structure called the phycobilisome, which contains a number of proteins that have covalently linked bilin chromophores (the biliproteins). We have utilized a protein engineering approach to study the structural determinants of chromophore attachment to the biliproteins. As the project developed, we discovered that disruption of the biliprotein subunit structure resulted in severe degradation in vivo. We have established that the earliest interactions in the phycobilisome assembly pathway are crucial to the stability of these proteins and must occur accurately and rapidly to avoid degradation. We have examined the role of chromophore attachment in protein stability and the data indicate that covalent attachment of the central bilins in both alpha and Beta subunits is needed to stabalize these proteins for assembly. We have used domain exchange experiments to examine chromophore attachment and have found three residues that appear to be required lor protein stability and chromophore attachment. We have developed a system for investigating the molecular basis of recognition and docking between biliprotein alpha and Beta subunits and have prelimina results from this work.