Phycobilisome linker polypeptide

The chlorophyll-protein (CP) and polypeptide pattern of PSII complexes were determined by two dimensional gel electrophoresis. Prominent polypeptides could be attributed to the biliprotein subunits of AP (14> 16 kDa) and PC (15f 18 kDa) and at least four phycobilisome-linker polypeptides (29f 31, 34 and 120 kDa. The Chl-proteins were characterized by apoproteins of 47 (CPIIa,b) and 41 (CPIIc) kDa. CP Ila and b were similar in their spectroscopic properties and showed fluorescence maxima at 685 nm and shoulders at 693 nm, whilst CP lie exhibited only one peak at 686 nm, when excited at 445 nm. Thus the isolated Chl-proteins had similar emission properties as the in situ" PSII antennae. 

Fig. 11. Phycobilisome of Masligocladus laminosus (Fischerella PCC 7603) represented in a schematic view. The structure is adapted from the PBS of Synechocystis 6701 [77] using structural and spectral data from M. laminosus biliproteins and linker polypeptides [22,31,82,97,105,135-137]. For the designation of the rod and core substructures, the nomenclature of Glazer [86] is applied.

In a medium of high ionic strength, in vitro hexamers can build up into short rods. However, such aggregation appears to require the presence of a so-called linker polypeptide (symbol L ), which apparently serves as a kind of glue. Since it has been shown that phycobiliproteins purified to strict homogeneity cannot form these phycobilisome subassemblies. The presence ofthese linker polypeptides in... 

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...

Fig. 9. Schematic representation of the phycobiiisome from Anabaena variabilis showing the location of the billproteins and linker polypeptides. See text for details. Adapted from Glazer (1984) Phycobilisomes A macmmolecular complex optimized for light energy transfer. Biochim Biophys Acta 768 42.

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.

Fig. 12. (A) Time-resolved fluorescence spectra of A. nidulans phycobilisomes measured at 77 K. Excitation by 6-ps, 580-nm argon laser pulse. Three small ticks in the topmost spectrum (at 932 ps) indicate locations of maximum fluorescence at 0 ps. (B) Rise and decay of various fluorescent components derived from deconvolution of the fluorescence spectra. Assignment of individual fluorescent components are shown in the right margin. (C) Energy flow among individual chromophores in the phycobilisomes. The asterisk in (B) and (C) indicates a linker polypeptide is attached to the trimer. See text for discussion. Figure source Mimuro (1989) Studies on excitation energy How in the photosynthetic pigment system structure and energy transfer mechanisms. Bot Mag Tokyo 103 244.

Copies per phycobilisome. Numbers reflect measured values for the ratio AP PC, and the relative ratios of the linker polypeptides. The actual numbers for the oAP-B and 318 subunits could be slightly higher than listed. These numbers reflect the composition of an idealized PBS as predicted by the model of Glazer (2) for a PBS with a tricylindrical core with a 1 2 ratio of AP PC. Data from (10). 

Based on earlier studies of the rod and core substructures the following model for the phycobilisome (PBS) of M. laminosus was proposed (1) (Fig. 4..A) the rods consist of one phycoerythrocyanin (PEC) and three phycocyanin (PC) hexamers (disks) connected by noncoloured linker polypeptides of 34 5i 31 5 and 29 5 kDa (app.MVT) and of a 8.9 kDa rod-terminator polypeptide located at the PEC hexamer the PEC-formation increases at lower light intensity the order of the hexamer-linker complexes in the PBS rods from outside to inside was believed to be... 

Fig. 1. SDS-PAGE of phycobilisomes and phycocyanin-linker polypeptide complexes. Lrc is partly degraded by a natural proteinase.

Fig. 4. Previous (A) and updated (B) model of the phycobilisome of Mastigocladus laminosus showing the location and polypeptide composition (phycobiliproteins, linker polypeptides) of the complexes making up the r s and the three-cylinder core. (1) Lr -, ...

The linker polypeptide (Lem) is supposed to bind the phycobilisome to the Chl antennae of PSII. Energy is funneled from the terminal emitters of the phycobilisome (Lem, AP B) to the antennae of PSII (CP 41, 47) and from there to the reaction centre. 

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