Bilin organization in cryptomonad biliproteins

The bilin organization of three cryptomonad biliproteins (phycocyanins 612 and 645 and phycoerythrin 545) was examined in detail. Two others (phycocya nin 630 and phycoerythrin 566) were studied less extensively. Phycocyanin 6 45 and phycoerythrin 545 were suggested to have one bilin in each monomeric (alpha beta) unit of the dimer (alpha(2)beta(2)) isolated from the others, and the remaining six bilins may be in pairs. One pair was found across th e monomer-monomer interface of the protein dimer, and two identical pairs w ere proposed to be within the monomer protein units. For phycocyanin 612, a major surprise was that a pair of bilins was apparently not found across t he monomer-monomer interface, but the remaining bilins were distributed as in the other two cryptomonad proteins. The effect of temperature on the CD spectra of phycocyainin 612 demonstrated that two of the bands (one positiv e and one negative) behaved identically, which is required if they are coup led. The two lowest-energy CD bands of phycocyanin 612 originated from pair ed bilins, and the two higher-energy bands were from more isolated bilins. The paired bilins within the protein monomers contained the lowest-energy t ransition for these biliproteins, Using the bilins as naturally occurring r eporter groups, phycocyanin 612 was shown to undergo a reversible change in tertiary structure at 40 degrees C. Protein monomers were shown to be func tioning biliproteins, A hypothesis is that the coupled pair of bilins withi n the monomeric units offers important advantages for efficient energy migr ation, and other bilins transfer energy to this pair, extending the wavelen gth range or efficiency of light absorption.