Far-red light photoacclimation: Chromophorylation of FR induced α- and β-subunits of allophycocyanin from Chroococcidiopsis thermalis sp. PCC7203

• Published in: Biochimica et biophysica acta 2016-09, Vol.1857 (9), p.1607-1616
• Main Authors: Xu, Qian-Zhao, Han, Jia-Xin, Tang, Qi-Ying, Ding, Wen-Long, Miao, Dan, Zhou, Ming, Scheer, Hugo, Zhao, Kai-Hong
• Format: Article
• Language: English
• Subjects: Adaptation, Physiological; Allophycocyanin; Binding Sites; Chromophorylation; Cyanobacteria; Cyanobacteria - chemistry; Energy transfer; Far-red photoacclimation; Fluorescence; Light; Phycobilisome; Phycocyanin - chemistry; Phycocyanobilin; Protein Subunits
• ISSN: 0005-2728; 0006-3002; 1879-2650
• DOI: 10.1016/j.bbabio.2016.06.008

Cyanobacterial light-harvesting complexes, phycobilisomes, can undergo extensive remodeling under varying light conditions. Acclimation to far-red light involves not only generation of red-shifted chlorophylls in the photosystems, but also induction of additional copies of core biliproteins that have been related to red-shifted components of the phycobilisome (Gan et al., Life 5, 4, 2015). We are studying the molecular basis for these acclimations in Chroococcidiopsis thermalis sp. PCC7203. Five far-red induced allophycocyanin subunits (ApcA2, ApcA3, ApcB2, ApcB3 and ApcF2) were expressed in Escherichia coli, together with S-type chromophore-protein lyases and in situ generated chromophore, phycocyanobilin. Only one subunit, ApcF2, shows an unusual red-shift (λAmax~675nm, λFmax~698nm): it binds the chromophore non-covalently, thereby preserving its full conjugation length. This mechanism operates also in two Cys-variants of the induced subunits of bulky APC. All other wild-type subunits bind phycocyanobilin covalently to the conventional Cys-81 under catalysis of the lyase, CpcS1. Although three of them also show binding to additional cysteines, all absorb and fluoresce similar to conventional APC subunits (λAmax~610nm, λFmax~640nm). Another origin of red-shifted complexes was identified, however, when different wild-type α- and β-subunits of the far-red induced bulky APC were combined in a combinatorial fashion. Strongly red-shifted complexes (λFmax≤722nm) were formed when the α-subunit, PCB-ApcA2, and the β-subunit, PCB-ApcB2, were generated together in E. coli. This extreme aggregation-induced red-shift of ~90nm of covalently bound chromophores is reminiscent, but much larger, than the ~30nm observed with conventional APC. •Two mechanisms account for far-red acclimation of light-harvesting complexes in cyanobacteria.•FR induced bulky allophycocyanin subunits covalently bind bilins to 1 or 2 cysteines.•Co-generation of α- and β-subunits, ApcA2 and ApcB2, results in 90nm red-shift.•With the β-18 subunit, ApcF2, red-shift is based on non-covalent chromophore binding.•Red-shifts of the phycobilisome complement induction of red-shifted chlorophylls.