A MODEL FOR EARLY EVENTS IN THE ASSEMBLY PATHWAY OF CYANOBACTERIAL PHYCOBILISOMES

Biological self-assembly is remarkable in its fidelity and in the effi cient production of intricate molecular machines and functional materi als from a heterogeneous mixture of macromolecules. The phycobilisome, a light-harvesting structure of cyanobacteria, presents the opportuni ty to study an in vivo assembly process in detail. The phycobilisome m olecular architecture is defined, and crystal structures are available for all major proteins, as are a large sequence database (including a genome sequence) and effective genetic systems exist for some cyanoba cteria. Recent studies on subunit interaction, covalent modification, and protein stability suggest a model for the earliest events in the p hycobilisome assembly pathway. Partitioning of phycobilisome proteins between degradation and assembly is proposed to be controlled by the i nteraction equilibria between phycobilisome assembly partners, process ing enzymes and chaperones. The model provides plausible explanations for existing observations and makes predictions that are amenable to d irect experimental investigation.