Phycobilisome assembly

When complete cells are treated with levulinic acid, a set of 11 peptides are lost from the membrane fraction. The same is true for recovering cells. Since the phycobiliproteins figure so prominently in this set, phycobilisome assembly must be extremely sensitive to levulinic acid. Yamanaka et al. (1978) investigated the phycobilisome structure of a related A. nidulans strain and observed 5 non-pigmented polypeptides. Interestingly, a several peptides lost by inhibition (71, 35, and 27 kDa) are similar in molecular size to peptides observed in the phycobilisome. 

Zilinskas BA and Howell DA (1983) Role of the colorless polypeptides in phycobilisome assembly in Nostoc sp.. Plant Physiol. 71, 379-387. 

FIGURE 19-43 A phycobilisome. In these highly structured assemblies found in cyanobacteria and red algae, phycobilin pigments bound to specific proteins form complexes called phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). The energy of photons absorbed by PE or PC is conveyed through AP (a phycocyanobilin-binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text. 

As seen earlier in Fig. 2, a phycobilisome is a fan-shaped set ofsupramolecular assemblies, consisting of a triangular core with six short rods radiating outward, with each phycobilisome being attached to the thylakoid membrane through the core complex. In crude extracts of cyanobacteria, these short rods are seen in electron micrographs to have a diameter of 12 nm, as expected for stacks of hexamers, with clearly marked divisions 6 nm apart along the rod and faint divisions midway between. 

Fig. 8. Representations for the phycobilisome rod components and linker polypeptides (A) and mechanism of assembly (B). Figure adapted from Mdrschel and Rhiel (1987) Phycobilisomes and thylakoids The light-harvesting system of cyanobacteria and red algae. In JR Harris and RW Horne (eds) Membranous Structure, p 238. Acad Press drawn after results of Glazer (1982) Phycobilisomes structure and dynamics. Annu Rev Microbiol 36 183...

Extensive fractionation of the dissociation products from A. variabilis phycobilisomes led to the isolation of three PC complexes, (a(3)3-LRc , (aP)3-LR - and (aP)3 LR , and one phycoerythrocyanin complex (aP)3 L °. With suchtrimer-linker complexes available, in vitro reconstitution experiments could be performed to help clarify the sequence of the components in the rods and how the linker polypeptides mediate rod assembly. As already shown in Pig. 8, a large portion of the linker polypeptide is buried inside the trimer, and only the small segment ofthe linker that is projecting outside the trimer is apparently able to link with the next trimer or hexamer. 

Fig. 10. Aggregation behavior of Anabaena variabilis phycocyanin compiexes with the 32.5- and 27-kDa linker poiypeptide after tryptic degradation. Original data from Yu and Glazer (1982) Cyanobactehal phycobilisomes. Roles of the linker polypeptides in the assembly of phycocyanin. J Biol Chem 257 3430 and Glazer (1982) Phycobilisomes structure and dynamics. Annu Rev Microbiol 36 189. Figure drawn in a form similar to that of Fig. 8, using the graphic representation of MSrschel and Rhiel.

DJ Lundell, RC Williams and AN Glazer (1981) Molecular architecture of a light-harvesting antenna. In vitro assembly of the rod substructures of Synechococcus 6301 phycobilisomes. J Biol Chem 256 3580-3592 S Brody and E Rabinowitch (1957) Excitation lifetime of photosynthetic pigments. Science 125 555 G Tomita and E Rabinowitch (1962) Excitation energy transfer between pigments in photosynthetic cells. Biophys J 2 483-499... 

Cyanobacteria and red algae contain protein assemblies called phycobilisomes, which absorb some of the green and yellow light that reaches them to perform photosynthesis. 

Phycobilisomes are large complex assemblies of phyco> biliproteins designed to collect light and transfer the energy to the photosynthetic reaction centers of cyanobacteria. Phy> cobilipFOteins and phycobilisomes contain several fluorescent species and are known to display complex intensity decays. While these complex decays can be analyzed in terms of the muitiexponential model, it is equally probable... 

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