Linker polypeptides of the phycobilisome from the cyanobacterium Mastigocladus laminosus. II. Amino-acid sequences and functions.

The complete primary structure of an 80-residue linker polypeptide, LR(C)8.9, from the phycobilisome of the cyanobacterium Mastigocladus laminosus was determined as well as the 44 N-terminal residues of the two linker polypeptides LR34.5,PEC and LR34.5,PC and the 114 C-terminal residues of LR34.5,PEC. A brief description of the structure determination and an extensive discussion of the relationships of these polypeptides have been published recently (Füglistaller, P., Suter, F. & Zuber, H. (1985) Biol. Chem. Hoppe-Seyler 366, 993-1001). In this paper we report in detail about the elucidation of the primary structures. Limited digestion of the hexameric phycobiliprotein-linker polypeptide complex (alpha PEC beta PEC)6LR34.5,PEC with various proteases resulted in a linker polypeptide diminished by a 1-5 kDa segment, while the phycobiliproteins remained intact. By N-terminal sequence analysis of the residual part of the linker polypeptide in the complex, LR34.5-5,PEC, it was concluded that the C-terminus of the polypeptide had been attacked by the proteases. This C-terminal part of the protein influences the hexamer formation of phycoerythrocyanin (PEC) and is responsible for the linkage between two phycobiliprotein hexamers. From the function of the C-terminal segment of LR34.5,PEC and its homology to the LR(C)8.9 polypeptide, it was concluded that LR(C)8.9 is located at the end of the peripheral phycobilisomal rods distal to the allophycocyanin core.