Ribbon representation of the preliminary crystal structure of PC645.

For photosynthetic organisms growing in poor light environments, it is critical that every photon reaching the organism is captured. The light harvesting protein antenna is a quantum mechanical machine for capturing photons entering the organism and ensuring that most of this energy is utilised in the process of photosynthesis.

The cryptophytes Chroomonas sp CS24, Chroomonas CCMP644 and Chroomonas CCMP270 are a unicellular photosynthetic alga with a light harvesting system that is very distinct from all other algae and photosynthetic bacteria (called cyanobacteria). Instead of a very complex antenna system, cryptophytes have a peripheral antenna composed of a water-soluble protein: phycoerythrin PE545, phycoerythrin PE555 or phycocyanin PC645 (the numbers indicate the position of their absorption wavelength maxima in nanometers corresponding to the colour of proteins: red, pink and blue) and a core antenna that contains a membrane protein with bound chlorophylls (light harvesting complex LHC).

Red Light Harvesting Protein
I have established an international collaborative program to determine the mechanism of light harvesting in an unusual type of single-celled alga, the cryptophytes. So far this program has successfully characterised the structure and the function of light harvesting protein PE545 from cryptophte Chroomonas sp CS24. We have determined:
• the first crystal structure of a cryptophyte light harvesting protein, PE 545, at true atomic resolution (better than 1 Å);
• the optical characteristics of the photon funnel produced by PE545 via femtosecond laser spectroscopy in collaboration with the spectroscopy group at the University of Toronto;
• the most detailed characterisations of any light harvesting system - the combination of ultra high resolution x-ray crystallography and ultra fast laser spectroscopy in collaboration with the spectroscopy group at University of Toronto.

In July 2005 following an invitation from the 30th FEBS Congress Committee, I presented the results of my research work on cryptophyte’s light harvesting antennae. This was well received and as a result, I have established collaboration with Vrije Universiteit van Amsterdam in the Netherlands aiming to study the optical properties of cryptophyte light harvesting proteins

Pink and Blue Light Harvesting Protein
Research work on PC645 is quite advanced, with sufficient x-ray diffraction data to solve the atomic structure at 1.4Å resolution (see Figure of preliminary structure of PC645) while the work on PE555 is still in the protein purification stage.

The combined structural and spectroscopic characterisations of three related light harvesting proteins will provide a unique view of how protein structure alters the optical properties of a natural light harvesting system.
This will have impact on both photosynthetic biology and artificial light harvesting technology.

Krystyna Wilk