Allophycocyanin: trimers, monomers, subunits, and homodimers.

Allophycocyanin is a photosynthetic light-harvesting pigment-protein complex located in the phycobilisomes of cyanobacteria and red algae. Using dynamic light scattering and circular dichroism, solutions of purified allophycocyanin were shown to consist of homogeneous trimers (alpha3beta3) with a nonspherical shape over a very wide range of protein concentrations at pH 6.0 and 20 degrees C. Deconvolutions of the visible circular dichroism spectrum of the trimer were carried out for the first determination of the individual spectra of all six-component chromophores. The chromophores were shown to be in different microenvironments that helped determine the spectrum of the trimer. Monomers (alpha beta) that were formed in either the presence of 0.50M NaSCN or at 45 degrees C were shown to be completely reversible to trimers. However, subunits (alpha and beta) that were formed in either the presence of 8M urea or at 60 degrees C, using spectroscopy and gel-filtration column chromatography, were observed to only partially reconstitute trimers. Homodimers (alpha2 and/or beta2) formed during the regeneration of trimers. The homodimer, which was detected for the first time when both subunits were present, was shown to be in equilibrium with its subunits. Unlike the trimer situation, subunits were found to fully reconstitute monomers in the presence of 0.50M NaSCN. These results suggest a route to trimer assembly from subunits with monomers serving as intermediaries and the homodimers forming in a nonproductive step that did not interfere with the overall assembly scheme.