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 UV-visible absorption properties for solar energy capture.
We present a new PBP crystal structure of PE566 from Cryptomonas pyrenoidifera CCAP 979/61 at 2.0Å and compare it to our previously published structures. This structure contains two chromophores–bilin 618 and bilin 584 that have not been observed in a crystal structure previously.
Comparing various cryptophyte PBP structures we have learnt that:
- Cryptophyte algal 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.
- Complexes can assemble into two distinctly different quaternary structural arrangements, termed ‘open’ and ‘closed’. The nature of the quaternary structure is dictated by the presence or absence of a single amino acid insertion within the α subunits. 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 exhibit strongly coupled central chromophores which produces quantum coherence and increased energy transfer rates.
- 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.
- The spectral properties of each complex are also defined by the type of linear tetra-pyrroles bound to the protein. In the structure presented here, two new chromophores are described.
Our observations are consistent with the theory that both the large number of α-subunit genes encoded in cryptophyte algae and the variety of linear tetrapyrroles that can be manufactured and utilised provide biology with the tools required to finely tune cryptophyte phycobiliproteins to optimize light harvesting in different conditions.