Sonani Ravi Raghav, Gupta Gagan Deep, Madamwar Datta, Kumar Vinay
BRD School of Biosciences, Vadtal Road, Satellite Campus, Sardar Patel University, Vallabh Vidyanagar, India.
Protein Crystallography Section, Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, India.
PLoS One. 2015 Apr 29;10(4):e0124580. doi: 10.1371/journal.pone.0124580. eCollection 2015.
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.
为了更深入地了解这种光捕获复合物的结构 - 功能关系，分离的藻胆体（PBS）亚组件已广泛用于X射线晶体学分析。别藻蓝蛋白（APC）是始终存在于PBS核心复合物中的藻胆蛋白。共价结合到APC的α和β亚基保守半胱氨酸残基上的藻蓝胆素（PCB）发色团，负责从藻蓝蛋白吸收太阳能并将其转移到光合装置。在已知的APC结构中，α和β亚基的异二聚体（称为αβ单体）组装成三聚体或六聚体。我们在此首次报道了从海洋蓝藻（席藻属A09DM）中分离出的APC的晶体结构。该晶体结构已根据所有观察到的数据精修至2.51 Å的分辨率，Rwork（Rfree）为0.158（0.229），原子模型具有良好的立体化学性质。席藻蛋白在溶液和晶格中以αβ单体的三聚体形式存在。席藻蛋白α和β亚基的整体三级结构以及三聚体四级折叠与其他已知的APC结构相似。此外，海洋APC中两个共价结合的PCB发色团的构型和构象与在淡水蓝藻和海洋红藻中观察到的相同。然而，更多的疏水残基构成了与席藻蛋白α亚基结合的发色团的环境，这主要是由于海洋蛋白中的氨基酸取代。
