A M _r 95,000 polypeptide in Porphyridium cruentum phycobilisomes and thylakoids: Possible function in linkage of phycobilisomes to thylakoids and in energy transfer

Abstract

Two pigmented polypeptides with the same molecular weight (Mr 95,000) were isolated from the photosynthetic apparatus of Porphyridium cruentum by sodium dodecyl sulfate/ polyacrylamide gel electrophoresis. A blue polypeptide from phycobilisomes had absorption and fluorescence emission spectra similar to those of allophycocyanin. A green-pigmented polypeptide from photosynthetic membranes (free of phycobilisomes) contained chlorophyll a. Several properties were common to the Mr 95,000 polypeptides from both sources: (i) identical molecular weights, (ii) identical gel electrophoresis patterns after limited protease digestion, and (iii) immunological crossreactivity with an IgG fraction directed against the Mr 95,000 polypeptide from phycobilisomes. On the basis of this evidence, a common polypeptide exists in phycobilisomes and thylakoids, and it probably anchors the phycobilisome to the thylakoid membrane. The fluorescence emission overlap of the blue and green polypeptides suggests that they are involved in the transfer of energy from phycobilisomes to thylakoids.Phycobilisomes are organelles on the photosynthetic membrane of red and blue-green algae (cyanobacteria) that function in absorbing light in the wavelength region where chlorophyll absorption is low. Such an extension of the absorption region especially benefits organisms growing under light-limiting conditions. Light absorbed by the phycobiliproteins (phycoerythrin, phycocyanin, and allophycocyanin) is funnelled through a long-wavelength-emitting allophycocyanin to chlorophyll a (reviewed in ref. 1). In cells ofthe red alga Porphyridium cruentum, Duysens (2) and French and Young (3) demonstrated energy transfer from phycoerythrin to chlorophyll. Because the transfer efficiencies from phycobiliproteins to chlorophyll are high (80-90%), it is expected that phycobilisomes have a functional attachment site in close proximity to photosynthetic reaction centers (4). Although phycobilisome attachment molecule(s) have not yet been found, such molecules have been identified for another thylakoid binding complex, the coupling factor (5). Assuming that there are specific attachment molecule(s) in phycobilisomes, we have undertaken a comparison of the polypeptide compositions of isolated phycobilisomes and of thylakoids (free of phycobilisomes) to determine which components are likely to be involved in phycobilisome/thylakoid attachment and energy transfer.The phycobilisome protein composition ofP. cruentum (synonymous with P. purpureum) has been reported (6). As with many other algae (7-9), the phycobilisomes were found to consist of many polypeptides, some ofwhich appeared colored and others noncolored after NaDodSO4/polyacrylamide gel electrophoresis. A pigmented polypeptide (F680) was reported to have a high molecular weight (95,000) and to constitute approximately 1.3% of the phycobilisome protein on the basis of Coomassie blue staining (6). As already reported in a preliminary communication (10), a polypeptide with the same molecular weight and a si...

Diffusion of Phycobilisomes on the Thylakoid Membranes of the Cyanobacterium Synechococcus 7942

“…The ApcE protein or "anchor polypeptide" of the phycobilisome core is implicated in the interaction with the membrane (7-9). Proposals for the association of ApcE with the membrane have included an integral membrane domain (7) or a covalently attached acyl group (11). However, we found that cooling below the phase transition temperature of the membrane had no significant effect on the mobility of phycobilisomes (Fig.…”

“…Phycobilisomes are largely composed of hydrophilic protein subunits (1,2). The ApcE protein, a high molecular weight component of the phycobilisome core, seems to play a crucial role in phycobilisome-membrane association (7,8). However, sequence analysis did not reveal any likely membrane-spanning domain (9), although phase partitioning suggests a hydrophobic region on the protein (10).…”

Evidence for a Transient Association of New Proteins with the Spirulina maxima Phycobilisome in Relation to Light Intensity

“…Linkers in core and rods (Lc and LR, respectively) ensure stability of disks (Glazer, 1985). Attachment of PBS on thylakoids (Redlinger and Gantt, 1982) and stability of the core are attributed to a (Isono and Katoh, 1987;Capuano et al, 1993).…”

Changes in Accumulation and Synthesis of Transcripts Encoding Phycobilisome Components during Acclimation of Fremyella diplosiphon to Different Light Qualities

“…The interactions of the biliproteins with the nonpigmented linker polypeptides (47) cause minor shifts in the spectral properties of the associated chromophores and may increase the efficiency of energy transfer between hexamers (18)(19)(20). Moreover, specific phycobiliproteins in the core of the phycobilisome (35,36,44) serve as energy bridges between the AP and the reaction center Chl of PSII (18,38,39).…”