Assembly of allophycocyanin from individual subunits: involvement of apo-form in the formation of trimers.

In cyanobacteria and red algae, allophycocyanin (APC), as well as other phycobiliproteins, is involved in the energy transfer of photosystems. Since APC is a potent fluorescent protein for imaging and biomedical applications, it is necessary to obtain purified protein in large quantities, which is currently possible by biosynthesis in bacterial systems. Here we emphasize the challenges of obtaining the trimeric form of the protein from α-APC and β-APC subunits of allophycocyanin in vitro. This approach allowed us to study the individual subunits and to perform assembly of allophycocyanin trimers in vitro. Using different spectroscopic techniques, we detected the heterogeneity of the synthesized β-APC and showed the possibility that not only holo-forms may be involved in trimer formation. Data allowed us to provide additional arguments in favor of excitonic coupling of chromophores in APC trimers.