Characterization and structural properties of the major biliproteins of Anabaena sp.

Abstract

Studies are presented of the biliproteins of Anabaena sp. This filamentous cyanobacterium contains three major biliproteins. Whereas two of these, C-phycocyanin and allophycocyanin, are common to all cyanobacteria, the third, phycoerythrocyanin (gammamax approximately 568 nm) has hitherto not been described and its distribution among cyanobacteria appears to be limited. Anabaena variabilis and Anabaena sp. 6411 allophycocyanin, C-phycocyanin, and phycoerythrocyanin were purified to homogeneity and characterized with respect to molecular weight, isoelectric point, absorption spectrum and amino acid composition. The alpha and beta subunits of each of these proteins were also purified to homogeneity and characterized in the same manner. The tetrapyrrole chromophore content was determined for each of the proteins and subunits. The alpha subunit of phycoerythrocyanin carries a novel phycobiliviolin-like chromophore. This chromophore has not previously been detected in cyanobacterial biliproteins, but has been noted as a prosthetic group of a cryptophytan phycocyanin. Sedimentation equilibrium studies show that at pH 7.0, at protein concentrations of 0.2-0.6 mg/ml, allophycocyanin, C-phycocyanin and phycoerythrocyanin, each exists as a trimeric aggregate, (alphabeta)3, of molecular weight of approximately 105000. Structrual studies of microcrystals of these three biliproteins by electron microscopy and X-ray diffraction reveal a common plan for the construction of higher assembly forms. The major building block appears to be the trimer (alphabeta)3. It is proposed that this is a disc-like structure about 3.0 X 12.0 nm. The individual alpha or beta subunits are roughly spherical, 3 nm in diameter. Allophycocyanin trimers stack to form bundles of rods which form long needles. Both phycocyanin and phycoerythrocyanin form double discs (alphabeta)6 which are visible as ring-shaped structures by electron microscopy. The mode of assembly of the biliprotein structures in the phycobilisome is, as yet, unknown.

Effects of Chromatic Illumination on Cyanobacterial Phycobilisomes

“…Polyacrylamide gel electrophoresis in the presence of urea and densitometry revealed the presence of two subunits in equimolar amounts carrying equal amounts of phycocyanobilin chromophores. A single species with a sedimentation coefficient of 5.2 S (50 mM sodium acetate buffer, pH 5.5) was detected in the analytical ultracentrifuge; thus, the purified allophycocyanin existed in solution as the trimer (@)3 [30,36,40]. Under conditions similar to those described for the crystallization of Anahaena 641 1 allophycocyanin [36], the Pseudanabaena 7409 allophycocyanin also crystallized as needles.…”

Phycoerythrocyanin and Phycoerythrin: Properties and Occurrence in Cyanobacteria

“…Phycoerythrocyanin is a cyanobacterial phycobiliprotein functionally similar to phycoerythrin but bearing some structural similarities to phycocyanin (Bryant et al, 1976(Bryant et al, , 1978Fueglistaller et al, 1980). The absorption properties of phycoerythrocyanin can effectively extend to shorter wavelengths the light-harvesting range of organisms containing this biliprotein (Table 1).…”

Biliproteins

“…In contrast, the pigments of the different spectroscopic types do not undergo cross reactions, even if they are produced from the same organism. On this basis, R-PC* 721 and phycoerythrocyanin* 32,2301 , which contain blue chromophores (la) as well as red chromophores (2), were classified as phycocyanins, and APC-B as a true allophycocyanin* 148b) . The subunits of individual C-PE* 2311 and also those of APC* 1491 -which are difficult to differentiate by other methods-are immunochemically not related, in con trast to the close relationship of corresponding subunits of pigments from other different organisms.…”

“…Starting from monomers, preferentially tri-and hexamers are formed* 2381 . These are the basic building blocks both for the crystalline pigments* 32,194,2191 as well as for phycobili somes* 223 2251 , as the light harvesting superstructures of these organisms* 51 . Dependent on the conditions of isolation, high er aggregates* 23,2391 and hetero aggregates* 223,224,2401 have also been found, which represent more or less intact fragments of phycobilisomes* 51 and even aggregates thereof 12251 .…”