Studies on the Dissociation of Cryptomonad Biliproteins

Robert Maccoll,Henry Malak,John Cipollo,Bryan Label,Glenn Ricci,Daniel Maccoll,Leslie E Eisele

doi:10.1074/jbc.270.46.27555

Abstract

The spectroscopic properties of two biliproteins, phycocyanin 645 and phycoerythrin 566, have been studied by treating the proteins with two different agents, NaSCN at pH 6.0, or pH 4.0 without NaSCN. For phycoerythrin 566, treatment with NaSCN revealed that the visible CD spectrum of its chromophores was separated into a pair of different spectra, and each of these spectra was observed as a negative and one or more positive bands. For phycocyanin 645, two negative CD bands have been observed previously, together with two or more positive bands, in the dimer (alpha 2 beta 2) state, and NaSCN treatment caused elimination of one of these negative bands. The dimer was stable at pH 6.0, but at pH 4.0 the spectra of phycocyanin 645 had one less negative band than those at pH 6.0. Chromatography demonstrated that phycocyanin 645 was a monomer (alpha beta) at pH 4.0. Monomers of cryptomonad biliproteins have never been observed before. Excitation at 514 nm, in picosecond time-resolved fluorescence studies, produced lifetimes of 11.0 and 45.2 ps for dimers and monomers, respectively. Excitation at 566 nm yielded times of 1.38 and 1.24 ps, for dimers and monomers, respectively. CD in the far UV showed that monomers and dimers had very similar secondary structures. These results have been used to test an hypothesis that proposed two types of exciton splitting among the chromophores of phycocyanin 645, and perhaps phycoerythrin 566 could also have this chromophore organization.

Highlights

Biliproteins are chromoproteins found in cyanobacteria, red algae, and cryptomonads
The positions and orientations of the chromophores for some biliproteins of the cyanobacteria and red algae are known (8 –13), and these determinations together with spectral studies have led to progress in understanding functions of the individual chromophores (14 –22)
Results (Figs. 1 and 3; Table I) obtained for phycoerythrin 566 suggested that two different pairs of chromophores might be involved in exciton splitting

Summary

Introduction

Biliproteins are chromoproteins found in cyanobacteria, red algae, and cryptomonads. In the two former types, biliproteins are organized into phycobilisomes, but phycobilisomes do not occur in the cryptomonads. Light absorbed by biliproteins migrates from the site of absorption to the reaction center of photosystem II through a network of pairwise transfers of energy These transfers are usually considered to occur via Forster resonance processes The positions and orientations of the chromophores for some biliproteins of the cyanobacteria and red algae are known (8 –13), and these determinations together with spectral studies have led to progress in understanding functions of the individual chromophores (14 –22). These investigations are directed at a study of cryptomonad biliproteins by carefully dissociating their dimeric structure. For phycocyanin 645, there have been studies on its spectroscopic properties (26 –30)

Methods

Results

Conclusion