X-ray structure of PE555 from the Hemiselmis rufescens CCMP644 at 2.0Å. The two a chains (green & blue) identical (ab) ‘monomer’ (i.e. blue & red) are structurally identical to PE545 & PC645. The quaternary structure is very different.

The photosynthetic algae species, cryptophytes, have evolved unique photosynthetic systems, which combine the use of water-soluble and membrane light-harvesting proteins.

The light harvesting systems in the cryptophytes are distinct from all other algae and cyanobacteria. Their peripheral antennae consist of one kind of phycobiliproteins and core antennae consist of protein bound chlorophylls. The chlorophylls absorb solar energy at 670 and 440 nm and phycobiliproteins absorb in between, allowing cryptophytes to increase the light harvest efficiency and live at lower light-regimes and greater depths than most other algae.

Current knowledge of the mechanisms involved in the light harvest in cryptophytes is limited. No definite overall model of energy transfer has been produced, despite the availability of ultrahigh resolution x-ray crystal structures of cryptophyte phycobiliproteins.

Preliminary analysis of the recently solved high resolution x-ray crystal structures of cryptophyte phycocyanin (PC645) of cryptophyte Chroomonas CCMP270, and phycoerythrin (PE555) of cryptophyte Hemiselmis rufescens CCMP644 have proved that the structures are similar to PE545 (published by our group in 1999) on a gross scale and their full atomic structures could reveal their distinct absorbance characteristics.

Recent spectroscopic studies, greatly assisted by the knowledge of the structures, will lead to an overall model for light harvesting mechanism utilised by the cryptophyte algae.

Krystyna Wilk