Phycobiliprotein structure

WA Sidler (1995) Phycobilisome and phycobiliprotein structures. In DA Bryant (ed) Molecular Biology of Cyanobacteria, pp 139-216. Kluwer... 

There are three main classes of phycobiliproteins, differing in their protein structure, bilin content, and fluorescent properties. These are phycoerythrin, phycocyanin, and allo-phycocyanin (APC). There are two main forms of phycoerythrin proteins commonly in use B-phycoerythrin isolated from Porphyridium cruentum and R-phycoerythrin from Gastroclonium coulteri. There also are three main forms of pigments found in these proteins phycoerythrobilin, phycourobilin, and phycocya no bilin (Glazer, 1985). The relative content of these pigments in the phycobiliproteins determines their spectral properties. All of them,... 

Figure 23-24 (A) Stereoscopic view of a hexameric (a(3)3 phycobiliprotein. (B) The (3 subunit of the complex with two molecules of bound phyco-ery throbilin and one of phy-courobilin. From Chang et al.279 (C) Schematic representation of a phycobilosome of a strain of the cyanobacterium Anabaena. Each disk in the structure contains an (a(3)3 phycobiliprotein. The circles marked AP are cross-sections of rods, each one composed of about four disks of allophycocyanin (AP).

Figure 5-9. Structure of two phycobilins that act as important accessory pigments. Phycoerythrobilin has fewer double bonds in conjugation than phycocyanobilin, so its Xmax occurs at shorter wavelengths (Fig. 5-8). Phycobilins occur covalently bound to proteins that is, they are the chromo-phores for phycobiliproteins.

In addition to the examples discussed above, these various reversed-phase HPLC mapping procedures have subsequently found numerous other advocates. Selected recent achievements include application to human hemoglobin variants 9a, 182, 185), the a- and j8-chains of rat hemoglobin 186), polypeptide hormones 9a, 99, 163), porcine C5a anaphylatoxin 187), Aplysia neuroactive polypeptide 188), ACTH/jS-lipotropin precursors 97, 99, 189, 190), oncoproteins 157), chick liver dihydrofolate reductase 191), limulin 192), ATP-citratelyase 193), spore-specific proteins 194), cAMP-dependent protein kinases (/95, 1%), interferons 197), complement components 9a, 198), aj-macroglobulin 198a), lectins 199), phycobiliproteins 200), bovine mitochondrial-F, ATPase 201), collagens, tubulins, and other structural proteins 9a, 202). 

In the PBS rods the phycobiliprotein hexamers can be identified by high-resolution electron microscopy as discs, subdivided into two halfs (a/3-trimers) of 30 A thickness [80,143]. Deeper insight into the molecular structure of the trimers and hexamers was achieved by X-ray crystallographic analyses of biliproteins. In the last century, strikingly coloured phycocyanin and phycoerythrin crystals had already been observed by Molish [144]. Recently, several C-phycocyanins [145-147], B-phycoerythrin [147,148] and phycoerythrocyanin [149] have been crystallized... 

Phycobiliproteins, such as phycocyanin and phycoerythrin, are members of a family of fluorescent accessory, nonchlorophyll-based pigments found in cyanobacteria and eukaryotic algae. The phycobiliproteins have characteristic broad absorption profiles spanning 450-600 nm, emissions ranging 570-660 nm, and small stokes shifts see Fig. 3. The major structural subunits, phycoerythrobilin (PEB) or phycocyanobilin... 

Table 1. Examples drawn from four classes of phycobiliproteins, allophycocyanin (APC), phycocyanin (PC), phycoerythrocyanin (PEC), and phycoerythrin (PE), arranged in the order of decreasing wavelengths of their major absorption bands. and x " are the peak wavelengths of the principal (and minor) absorption and fluorescence bands, respectively. See List of Abbreviations for the full names of the different phycobiliproteins. Table adapted from Glazer (1982) Phycobilisomes Structure and dynamics. Annu Rev Microbiology 36 178 and Glazer (1989) Light guides. Directionai energy transfer in a photosynthetic antenna. J Biol Chem. 264 2.

X-ray crystallographic analysis of C-phycocyanin from Mastigocladus laminosus (a thermophilic cyanobacterium) shows the phycobiliprotein to be a doughnut-shaped trimer [see Fig. 7 below]. The a-and (3-isubunits have tertiary structures that are similar to one another, consistent with their sequence... 

Fig. 6. Top Schematic representation ofthe assembly of phycobiliprotein trimers and hexamers from the a- and p-subunits (same as Fig. 2). (A) Stereogram ofthe C-PC p-subunit (B) stereogram of the C-PC(ap)-monomer. Helices are represented by cylinders those ofthe p-subunit are labeled with uppercase letters and those ofthe a-subunit with lowercase letters. Chromophores in (A) and (B) are represented by wire models. Figure source (A) and (B) Schirmer, Bode, Huber, Sidler and Zuber (1985) X-ray crystallographic structure of the light-harvesting biliprotein C-phycocyanin from the thermophilic cyanobacterium Mastigocladus laminosus and its resemblance to globin structures. J Mol Biol 184 268,272.

The phycobiliproteins are hard to beat as extremely bright antibody labels for fluorescence analysis of cell surface antigens by flow cytome-try 15,16 These bacterial photosynthetic macromolecules each have up to 34 individual bilin fluorophores wrapped within the polypeptide structure. An example is R-phycoerythrin (R-PE), which has an extinction coefficient of 2 X 10 L/mol cm and a quantum yield of 0.68. Usually there is room for only one PE molecule per antibody, because the size of an R-PE label is 1.5 times that of an IgG antibody. The large size of the complex reduces the kinetics of binding to cell surface antigens, and some intracellular markers are inaccessible to the R-PE-labeled antibody. For intracellular measurements, lower molecular mass fluorophores (<1 kDa) are usually preferred. 

Although PBS occur in several structural variants, hemidiscoidal PBS are by far the most common. These structures are composed of either eight or nine cylindrical substructures which can be placed into two functionally defined categories. The "peripheral rods" are cylinders 11 nm in diameter of variable length (6-40 nm) and composition. These structures may be exclusively composed of the phycobiliprotein phyco-cyanin (PC, Xmax 620 nm), but may additionally contain phycoerythrin (PE, Xmax 560 nm) and phycoerythrocyanin (PEC, Xmax 570 nm) when pro-... 

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