Crystal Structure of Allophycocyanin from Marine Cyanobacterium Phormidium sp. A09DM

Ravi Raghav Sonani,Gagan Deep Gupta,Datta Madamwar,Vinay Kumar,Alexander Wlodawer

doi:10.1371/journal.pone.0124580

Ravi Raghav Sonani, Gagan Deep Gupta + Show 3 more

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https://doi.org/10.1371/journal.pone.0124580

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Journal: PloS one	Publication Date: Apr 29, 2015
Citations: 30	License type: CC BY 4.0

Affiliation: Sardar Patel University, Bhabha Atomic Research Centre

Abstract

Isolated phycobilisome (PBS) sub-assemblies have been widely subjected to X-ray crystallography analysis to obtain greater insights into the structure-function relationship of this light harvesting complex. Allophycocyanin (APC) is the phycobiliprotein always found in the PBS core complex. Phycocyanobilin (PCB) chromophores, covalently bound to conserved Cys residues of α- and β- subunits of APC, are responsible for solar energy absorption from phycocyanin and for transfer to photosynthetic apparatus. In the known APC structures, heterodimers of α- and β- subunits (known as αβ monomers) assemble as trimer or hexamer. We here for the first time report the crystal structure of APC isolated from a marine cyanobacterium (Phormidium sp. A09DM). The crystal structure has been refined against all the observed data to the resolution of 2.51 Å to Rwork (Rfree) of 0.158 (0.229) with good stereochemistry of the atomic model. The Phormidium protein exists as a trimer of αβ monomers in solution and in crystal lattice. The overall tertiary structures of α- and β- subunits, and trimeric quaternary fold of the Phormidium protein resemble the other known APC structures. Also, configuration and conformation of the two covalently bound PCB chromophores in the marine APC are same as those observed in fresh water cyanobacteria and marine red algae. More hydrophobic residues, however, constitute the environment of the chromophore bound to α-subunit of the Phormidium protein, owing mainly to amino acid substitutions in the marine protein.

Highlights

Phycobiliproteins (PBPs) and linker proteins (LPs) are mutually arranged in cyanobacteria and red algae to form multi-molecular assemblies of around 4–8 MDa called phycobilisomes (PBS) [1,2,3]
We report the first crystal structure of APC isolated from the marine cyanobacterium, Phormidium sp
SDS-Polyacrylamide gel electrophoresis (PAGE) profile of purified APC showed only two bands, which corresponded to αand β- subunits, and substantiated the purity of APC preparation

Summary

Introduction

Phycobiliproteins (PBPs) and linker proteins (LPs) are mutually arranged in cyanobacteria and red algae to form multi-molecular assemblies of around 4–8 MDa called phycobilisomes (PBS) [1,2,3]. Rods contain phycocyanin (PC, absorption λmax *610–620 nm) and/or phycoerythrin (PE, absorption λmax *540–570 nm), and associated LPs. The core invariantly contains allophycocyanin. Crystal Structure of a Marine Cyanobacterium Allophycocyanin Protein (APC, absorption λmax *653 nm) and associated LPs [3]. Phycobilisomes harvest the sunlight and exhibit efficient energy transfer in the direction from PE ! The energy absorbed by chlorophyll is used to split water molecules generating electrons and protons in the photosynthetic reaction centers. It has recently been shown that phycobilisomes supply energy to both the photosystems I and II [8]

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