Structural analysis at 2.2 Å of orthorhombic crystals presents the asymmetry of the allophycocyanin–linker complex, AP⋅L _C ^7.8 , from phycobilisomes of Mastigocladus laminosus

Abstract

An electrophoretically purified allophycocyanin-linker complex, AP⅐L C 7.8 , from phycobilisomes of Mastigocladus laminosus has been crystallized in the orthorhombic space group P2 1 2 1 2 1 . Cryocrystallographic x-ray measurements enabled the structural analysis of the complex at a resolution of 2.2 Å. The asymmetric unit contains two sideto-side associated ''trimeric'' (␣␤) 3 allophycocyanin complexes comprising the linker polypeptide in a defined orientation inside the trimer. The linker representing a protein fold related to the prosegment of procarboxypeptidase A is in contact with only two of the three ␤-subunits and directly interacts with the corresponding chromophores of these proteins. In addition to a modulation of the chromophores' spectral properties, the linker polypeptide attracts the ␣␤-subcomplexes, thereby bringing the ␤-chromophores closer together. These results will enable interpretations of energytransfer mechanisms within phycobiliproteins.

Biosynthesis of fluorescent cyanobacterial allophycocyanin trimer in Escherichia coli

“…Bilin modification, such as reduction and photobleaching, prevents the formation of in vitro aggregation states higher than dimers (Fischer and Scheer 1992). Bilin, when located in the E a helix, has extensive interaction with residue sidechains in the E-F a helices (Liu et al 1999;McGregor et al 2008;Murray et al 2007;Reuter et al 1999;Schirmer et al 1987). Therefore, bilin absence may alter local side-chain packing, leading to destabilized subunit interaction and disruption of subsequent assembly events ).…”

Structural organization of an intact phycobilisome and its association with photosystem II

“…The connections of these REP domains mediated by loops [36] cannot be clearly observed at the current resolution. The Pfam01383 domains are located inside the trimer plane, as in the crystal structure [10], whereas the Pfam00427 domains protrude from the trimer plane ( Figure 2B and Supplementary information, Figure S6A). APC trimers in the core (cylinders A1, A2, and B) are associated via a specific pattern, in which a corner of the triangle-shaped trimer directly contacts the edge of another trimer ( Figure 2B and 2C).…”

“…The crystal structure of APC in complex with the linker protein Lc (the Pfam01383 domain) from Mastigocladus laminosus (PDB code: 1B33) [10] and a recently reported crystal structure of the N-terminal domain of linker L R (the Pfam00427 domain) from Synechocystis sp. PCC 6803 (PDB code: 3NPH) [20] were used for docking analysis of the core.…”

“…The crystal structures of some isolated components of PBSs from different species have been determined. These include PEs [12,13], PCs [8,[14][15][16][17], APCs [10,[17][18][19], and linker proteins [10,20] (Supplementary information, Figure S1). The structural layouts of various PBSs have also been suggested in electron microscopic images [21,22].…”

“…The trimers are the fundamental assembly npg www.cell-research.com | Cell Research unit of PBSs, and stack face to face to form a hexamer with or without linker proteins in its central cavity [2,9]. APCs in the core have a few variants, such as α LCM (the α domain in L CM ; L CM is coded by apcE), α APC -like variant (denoted α AP-B , coded by apcD), and β APC -like variant (denoted β 18.5 , coded by apcF) (Supplementary information, Figure S1A) [10].…”