title: Insights into the biosynthesis and assembly of cryptophycean phycobiliproteins.
abstract: Phycobiliproteins are employed by cyanobacteria, red algae, glaucophytes, and cryptophytes for light-harvesting and consist of apoproteins covalently associated with open-chain tetrapyrrole chromophores. Although the majority of organisms assemble the individual phycobiliproteins into larger aggregates called phycobilisomes, members of the cryptophytes use a single type of phycobiliprotein that is localized in the thylakoid lumen. The cryptophyte Guillardia theta (Gt) uses phycoerythrin PE545 utilizing the uncommon chromophore 15,16-dihydrobiliverdin (DHBV) in addition to phycoerythrobilin (PEB). Both the biosynthesis and the attachment of chromophores to the apophycobiliprotein have not yet been investigated for cryptophytes. In this study, we identified and characterized enzymes involved in PEB biosynthesis. In addition, we present the first in-depth biochemical characterization of a eukaryotic phycobiliprotein lyase (GtCPES). Plastid-encoded HO (GtHo) was shown to convert heme into biliverdin IXα providing the substrate with a putative nucleus-encoded DHBV:ferredoxin oxidoreductase (GtPEBA). A PEB:ferredoxin oxidoreductase (GtPEBB) was found to convert DHBV to PEB, which is the substrate for the phycobiliprotein lyase GtCPES. The x-ray structure of GtCPES was solved at 2.0 Å revealing a 10-stranded β-barrel with a modified lipocalin fold. GtCPES is an S-type lyase specific for binding of phycobilins with reduced C15=C16 double bonds (DHBV and PEB). Site-directed mutagenesis identified residues Glu-136 and Arg-146 involved in phycobilin binding. Based on the crystal structure, a model for the interaction of GtCPES with the apophycobiliprotein CpeB is proposed and discussed.
authors.affiliation: Physiology of Microorganisms, Faculty for Biology and Biotechnology, Ruhr University Bochum, 44780 Bochum, Germany.
authors.forename: Kristina E
authors.initials: KE
authors.lastname: Overkamp
authors.affiliation: Protein Crystallography, Faculty for Biology and Biotechnology, and Ruhr University Bochum, 44780 Bochum, Germany.
authors.forename: Raphael
authors.initials: R
authors.lastname: Gasper
authors.affiliation: Physical Chemistry I, Protein Interactions, Faculty for Chemistry and Biochemistry, Ruhr University Bochum, 44780 Bochum, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Kock
authors.affiliation: Physical Chemistry I, Protein Interactions, Faculty for Chemistry and Biochemistry, Ruhr University Bochum, 44780 Bochum, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Herrmann
authors.affiliation: Protein Crystallography, Faculty for Biology and Biotechnology, and Ruhr University Bochum, 44780 Bochum, Germany.
authors.forename: Eckhard
authors.initials: E
authors.lastname: Hofmann
authors.affiliation: Physiology of Microorganisms, Faculty for Biology and Biotechnology, Ruhr University Bochum, 44780 Bochum, Germany. Electronic address: nicole.frankenberg@rub.de.
authors.forename: Nicole
authors.initials: N
authors.lastname: Frankenberg-Dinkel
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001664:Biliverdine / Q000031:analogs & derivatives / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010088:Oxidoreductases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010799:Phycoerythrin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010944:Plants / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D020524:Thylakoids / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D010799:Phycoerythrin; C071708:15,16-dihydrobiliverdin IXa; D010088:Oxidoreductases; D001664:Biliverdine
keywords: Algae; Bilin; Biosynthesis; Chromophore; Crystal Structure; Light Harvesting; Photosynthetic Pigment; Protein Assembly; Tetrapyrrole
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