We study the soluble light harvesting phycobiliprotein (PBP) antenna complexes from cryptophyte algae. Each PBP is comprised of a conserved structure of two αβ dimers with covalently attached linear-tetrapyrrole chromophores that provide strong visible absorption properties. We have reported previously that complexes assemble into two distinctly different quaternary structural arrangements, termed ‘open’ and ‘closed’.
We present several new PBP crystal structures and compare them to our previously published structures. We conclude two important points regarding the variation of the PBPs spectral properties and quaternary structures.
Firstly, very similar complexes with identical chromophores have significant differences in their absorption properties. Our data indicate that the α-subunits regulate the absorption spectra for each complex by applying structural and chemical restraints on the covalently-coupled chromophores. Thus, the α-subunits are largely responsible for tuning the absorption properties of each complex.
Secondly, the α-subunits controls the nature of the quaternary structure – dictating whether the complex adopts the open or closed form. We have identified a single amino acid insertion in the α-subunit that forces a large ~73˚ rotation between the two αβ-heterodimers giving rise to the ‘open’ form. This structural change between the open and closed form is significant for light harvesting function because it disrupts strong excitonic coupling between two central chromophores. The ‘closed’ forms are strongly coupled and show quantum coherence.
Cryptophyte algae genomes typically encode a single highly conserved β-subunit gene (which we believe carries out a conserved structural role in the complexes) and a number of different α-subunit genes. Our observations are consistent with the theory that the large number of α-subunit genes encoded are able to tune the complexes to the environmental conditions to optimize light harvesting in different conditions.