Phycobiliprotein hexamer

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... 

Fig. 4. Assembly of phycobiliprotein trimers and hexamers from a- and p-subunits. See text for discussion.

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.

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... 

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