Molecular assembly of the phycobilisomes from the cyanobacterium Mastigocladus laminosus

Abstract

Five phycocyanin- and two phycoerythrocyanin-associated linker polypeptides were resolved by two-dimensional gel electrophoresis from isolated phycobilisomes of the thermophilic cyanobacterium Mastigocladus laminosus. These linker polypeptides were located in the substructure of the phycobilisome by analysis of isolated high molecular weight allophycocyanin—phycocyanin (AP—PC) complexes and by examination of different phycobilisome “types” induced by alteration of the culture conditions. The core fractions reveal two different rod—core linker polypeptides L RC 29.5PC and L RC 32PC and one rod linker L R 31PC associating the first and the following “trimeric” PC complexes at the profile sides of the core. The “minimal” phycobilisome type with an apparent molecular weight M r of 4 500 000 is characterized by the occurrence of only one additional rod linker polypeptide L R 34.5PC. Further sequential elongation of the rods occurs by PC and phycoerythrocyanin (PEC) “trimers” in cooperation with two low molecular weight linker polypeptides L R 11PC and L R 11PEC. The structure of the “maximal” phycobilisome with M r 8 500 000 is completed by the association of PEC “hexamers” via L R 34.5PEC. This sequential assembly could be demonstrated by the polypeptide composition of different “intermediate” and “maximal” phycobilisomes. The large number of linker polypeptides and the stoichiometric calculations of core fractions and of different phycobilisome “types” indicate a partial “trimeric” organization of the rod complexes. With respect to polypeptide stoichiometry, the “trimeric” arrangement of PC·L RC 29.5/32, PC·L R 31, PC·L R 11 and PEC·L R 11 in the structure of the rods is proposed. The structural and functional consequences of the study are represented in a model of the “maximal” phycobilisome.