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Crystal structure of the Bilin-binding domain of phycobilisome core-membrane linker ApcE
Structural highlights
FunctionAPCE_NOSS1 This protein is postulated to act both as terminal energy acceptor (by its phycobilin-like domains) and as a linker polypeptide (by its repeats and arms) that stabilizes the phycobilisome core architecture. Has intrinsic bilin lyase activity (By similarity). Publication Abstract from PubMedPhotosynthesis relies on energy transfer from light-harvesting complexes to reaction centers. Phycobilisomes, the light-harvesting antennas in cyanobacteria and red algae, attach to the membrane via the multidomain core-membrane linker, LCM. The chromophore domain of LCM forms a bottleneck for funneling the harvested energy either productively to reaction centers or, in case of light overload, to quenchers like orange carotenoid protein (OCP) that prevent photodamage. The crystal structure of the solubly modified chromophore domain from Nostoc sp. PCC7120 was resolved at 2.2 A. Although its protein fold is similar to the protein folds of phycobiliproteins, the phycocyanobilin (PCB) chromophore adopts ZZZssa geometry, which is unknown among phycobiliproteins but characteristic for sensory photoreceptors (phytochromes and cyanobacteriochromes). However, chromophore photoisomerization is inhibited in LCM by tight packing. The ZZZssa geometry of the chromophore and pi-pi stacking with a neighboring Trp account for the functionally relevant extreme spectral red shift of LCM. Exciton coupling is excluded by the large distance between two PCBs in a homodimer and by preservation of the spectral features in monomers. The structure also indicates a distinct flexibility that could be involved in quenching. The conclusions from the crystal structure are supported by femtosecond transient absorption spectra in solution. The terminal phycobilisome emitter, LCM: A light-harvesting pigment with a phytochrome chromophore.,Tang K, Ding WL, Hoppner A, Zhao C, Zhang L, Hontani Y, Kennis JT, Gartner W, Scheer H, Zhou M, Zhao KH Proc Natl Acad Sci U S A. 2015 Dec 29;112(52):15880-5. doi:, 10.1073/pnas.1519177113. Epub 2015 Dec 15. PMID:26669441[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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