Biochem. J. (1980) 187, 303-309 303 Printed in Great Britain The Native Forms of the Phycobilin Chromophores of Algal Biliproteins A CLARIFICATION Padraig O'CARRA,* Richard F. MURPHYt and S. Derek KILLILEAt Department ofBiochemistry, University College, Galway, Ireland (Received 3 September 1979) Pigments released from phycoerythrins and phycocyanins by treatment with hot methanol are currently regarded as equivalent to the native chromophores phyco- erythrobilin and phycocyanobilin. However, evidence presented here confirms the original views of O'Carra & O'hEocha [(1966) Phytochemistry 5, 993-9971 that these methanol-released pigments are artefacts differing in their chromophoric conjugated systems from the native protein-bound prosthetic groups. By contrast, the native spectral properties are retained in pigments released by careful acid treatment of the biliproteins and these acid-released phycobilins, rather than the methanol-released pigments, are therefore regarded as the protein-free forms of the native chromophores. The conclusion reached by Chapman, Cole & Siegelman [(1968) J. Am. Chem. Soc. 89, 3643-36451, that all the algal biliproteins contain only phycoerythrobilin and phycocyanobilin, is shown to be incorrect. The identification of a urobilinoid chromophore, phycourobilin, accompanying phycoerythrobilin in B- and R- phycoerythrins is confirmed and supported by more extensive evidence. The cryptomonad phycocyanins are shown to contain a phycobilin chromophore accom- panying phycocyanobilin. This further phycobilin has the spectral properties of the class of bilins known as violins and the provisional name 'cryptoviolin' is proposed pending elucidation ofits structure. The algal biliproteins are a group of brilliantly the phycocyanins is imparted by open-chain tetra- coloured fluorescent proteins that function as photo- pyrrole chromophores that are covalently attached synthetic pigments in red, blue-green and crypto- to the apoproteins (O'hEocha, 1965, 1966). These monad algae (O'hEocha, 1965, 1966). They are chromophores are termed phycobilins. They have subdivided into two major categories on the basis of labile structures and are difficult to release from the visual appearance: the phycoerythrins are red with a apoproteins without alteration. This has greatly golden fluorescence; the phycocyanins are blue with hindered the complete structural characterization of a red fluorescence (O'hEocha, 1965, 1966). Several the pigments. We believe that the structures cur- subtypes of these chromoproteins are distinguished rently accepted for the major phycobilins, phyco- on the basis of differences in absorption spectra. R-, erythrobilin and phycocyanobilin, apply to artefact B- and C-phycoerythrins and R-, C- and allo- derivatives of the native chromophores. Since the phycocyanins are found in various members of the confusion of native and artefact forms has been Rhodophyta (red algae) and Cyanophyta (blue- extensive (Beuhler et al., 1976; Chapman et al., green algae). Cryptomonad algae contain a complex 1967, 1968; Cole et al., 1967; Crespi & Katz, 1969; group of further variants [see O'hEocha et al. (1964) Crespi et al., 1968; Glazer & Fang, 1973; Glazer & for a review ofthese]. Hixson, 1975; Troxler et al., 1978), we decided that The brilliant coloration of the phycoerythrins and it was appropriate to clarify this issue before * To whom reprint requests should be addressed. proceeding to a consideration of the structures of the t Present address: Department of Biochemistry, native chromophores [see the following paper Medical Biology Centre, Queen's University, Belfast, (Killilea et al., 1980)]. Northern Ireland, U.K. A further issue needing clarification is the t Present address: Department of Biochemistry, North question as to whether phycoerythrobilin and phy- Dakota State University, Fargo, ND 58102, U.S.A. cocyanobilin are the only phycobilins occurring in Vol. 187 0306-3275/80/050303-07$01.50/1 © 1980 The Biochemical Society 304 P. O'CARRA, R. F. MURPHY AND S. D. KILLILEA algal biliproteins, as claimed by Chapman et al. (O'Carra et al., 1964; Riidiger & O'Carra, 1969). (1968). O'hEocha & O'Carra (1961) and O'Carra et Since the artefact attachment takes place by con- al. (1964) suggested the occurrence of a third type densation of the vinyl-side-chain group of ring D of of phycobilin with urobilinoid spectral properties phycoerythrobilin with thiol groups on the poly- in R-phycoerythrin, and evidence presented here peptide chains (O'Carra et al., 1964; Riidiger & strongly supports this. In addition, spectral evi- O'Carra, 1969), blocking of the thiol groups by dence suggest that, as well as containing phycocy- reduction and aminoethylation (Raftery & Cole, anobilin, cryptomonad phycocyanins have a further 1966) before tryptic digestion was carried out to type of phycobilin with violinoid spectral properties eliminate this complication. To further protect the (O'hEocha et al., 1964). We propose that these chromophores, all procedures used were carried out further phycobilins be termed respectively phyco- in the dark and under nitrogen. Tryptic digests were urobilin and cryptoviolin. carried out as follows. Reduced and aminoethylated biliprotein (50mg) was suspended in 10ml of Experimental 0.2M-sodium phosphate buffer, pH7.0. A solution Biliprotein preparations (0.1 ml) of diphenylcarbamoyl chloride-treated tryp- R-phycoerythrin from the red alga Rhodymenia sin (2.5 mg; Sigma) in 1 mM-HCl was added. The palmata, C-phycoerythrin from the red alga mixture was incubated with stirring for 6h at 250C Phormidium persicinum, C-phycocyanin from the under nitrogen, and any insoluble material was blue-green alga Nostoc punctiforme and R-phyco- removed by centrifugation. After the addition of a cyanin from the red alga Ceramium rubrum were further portion (0.1 ml) of trypsin solution to the isolated and purified as by O'Carra (1965). Crypto- supematant, digestion was continued for 16 h. monad phycocyanin was isolated from Hemiselmis Digests were stored at -200C under nitrogen. virescens as by O'hEocha et al. (1964). Starch-gel electrophoresis of tryptic digests of Bilin preparations R-phycoerythrin Methanol-released 'purple pigment' from R- Electrophoresis of tryptic digests of R-phyco- phycoerythrin and 'blue pigment' from C-phyco- erythrin was carried out on horizontal starch gels by cyanin were prepared as by O'Carra & O'hEocha using 'Starch Hydrolysed' (Connaught Medical (1966). Preparations of phycoerythrobilin and Research Laboratories, Toronto, Ont., Canada) and phycocyanobilin from R-phycoerythrin and C- adopting the procedure of Smithies (1955). The phycocyanin, respectively, were obtained by incu- buffer systems used were: glycine/HCl, pH 2.2; bating the biliproteins for successive periods of sodium acetate, pH4.0, 4.5, 5.0 and 5.5; Tris/mal- l5min at 180C in 11.5-12M-HCl (O'Carra et al., ate, pH 7.4 and 8.3. For the glycine/HCl and sodium 1964; O'hEocha, 1963). Phycocyanobilin was acetate buffers, the gel buffer concentration was similarly released from R-phycocyanin when most of 0.025M and the bridge buffer concentration was the phycoerythrobilin had been first released from 0.05M. For the Tris/malate buffers, the gel buffer the protein by two successive incubations with HCl concentration was 0.005M and the bridge buffer (O'Carra, 1962; Murphy, 1968). T.l.c. on silica-gel concentration was 0.1 M. Electrophoresis was carried G was used for analysis of the free diacid forms of out at 40C in the dark for 16h at 7.5 V/cm. After the phycobilin preparations and for purification of electrophoresis the starch gels were soaked in individual bilin constituents (Murphy, 1968). The ethanol/pyridine (1: 1, v/v) for 10min. The solvent systems used were: carbon tetra- bathing fluid was replaced for each of two subse- chloride/acetic acid (2: 1, v/v); carbon tetra- quent washings. The gels were then placed in a chloride/acetone/acetic acid (4:8:1, by vol.); hep- saturated solution of zinc acetate in ethanol for tane/acetone/acetic acid (16:16:3, by vol.). Bilins 10min to convert the bilin chromophores into their were esterified (Murray, 1966) in a solution (1 ml) of zinc complexes. Although the peptide bands that BF3 in methanol (14%, v/v; Sigma, Poole, Dorset, contained bilin chromophores could be detected U.K.) for 25min at 40C. Ice-cold water (4ml) was visually after electrophoresis, much greater sen- then added and the esterified pigment was extracted sitivity was achieved by viewing the fluorescent zinc with three 2ml portions of chloroform. complexes under u.v. light, when phycoerythrobilin and phycourobilin exhibit orange and green fluores- Tryptic digestion ofbiliproteins cence respectively. At each pH value used for Previous studies have shown that, under some electrophoresis of the tryptic digest, only one peptide conditions used to release phycoerythrobilin from containing phycoerythrobilin and one containing phycoerythrins, random artefact covalent linkage of phycourobilin could be detected. Similar results the bilin to the polypeptide chains of the apoprotein were obtained when the digests were subjected can take place and the chromophore is also to electrophoresis on polyacrylamide disc gels by converted into a urobilinoid artefact derivative the method of Dietz & Lubrano (1967). 1980 THE NATIVE PHYCOBILIN CHROMOPHORES OF ALGAL BILIPROTEINS 305 Results and Discussion released and methanol-released pigment prepara- tions. The two preparations retain their spectral To simplify discussion, the phycobilins are