Phycobiliprotein trimer

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.

Phycobiliproteins are found also in cryptophytes but, differently from cyanobacteria and red algae, they are not organized into a phycobilisome, but instead they are located in the thylakoid lumen. Unique for cryptophytes, their phycobiliproteins do not exhibit a trimeric aggregation state characteristic for cyanobacteria, but instead they are present as ai(3a2(3 heterodimers, with each a subunit having a distinct amino acid sequence. [40]... 

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... 

The wide variety ofways in which the subunits andthephycobilins are found combined in phycobiliproteins can be illustrated by three examples. In APC, each a- and P-subunit contains one PCB chromophore for a total of six for the (aP)3 trimer. In C-PC, each a-subunit contains one PCB chromophore and each p-subunit contains two, so a C-PC trimer contains altogether 9 PCBs. In R-PE there are six a- and six P-subunits and one y-protein subunit, i.e., (ap)6y. With each a-subunit in R-PE containing two PEBs, each P-subunit two PEBs and one PUB, and the one y- subunit containing two PEBs and two PUBs, there is a total of 34 phycobilin molecules per (aP)gy. 

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... 

Allophycocyanin from Mastigocladus laminosus is a blue water-soluble pigment protein-complex and a component of the phycobilisome core. The aggregates of core phycobiliproteins are trimeric complexes as shown below ... 

Phycobiliproteins associate as heterodimers of a and P monomeric subunits that, in turn aggregate into trimeric aP)i and hexameric discs aP)(,- The rod structure characteristic of PBSs is stabilized by non-pigmented linker polypeptides (L) specifically associated with each type of phycobiliprotein (Table 2) to optimize their absorption and energy transfer properties. 

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