Abstract
Phycobilisomes (PBS) are accessory light harvesting protein complexes that directionally transfer energy towards photosystems. Phycobilisomes are organized in a central core and rods radiating from it. Components of phycobilisomes in Gracilaria chilensis (Gch) are Phycobiliproteins (PBPs), Phycoerythrin (PE), and Phycocyanin (PC) in the rods, while Allophycocyanin (APC) is found in the core, and linker proteins (L). The function of such complexes depends on the structure of each component and their interaction. The core of PBS from cyanobacteria is mainly composed by cylinders of trimers of α and β subunits forming heterodimers of Allophycocyanin, and other components of the core including subunits αII and β18. As for the linkers, Linker core (LC) and Linker core membrane (LCM) are essential for the final emission towards photoreaction centers. Since we have previously focused our studies on the rods of the PBS, in the present article we investigated the components of the core in the phycobilisome from the eukaryotic algae, Gracilaria chilensis and their organization into trimers. Transmission electron microscopy provided the information for a three cylinders core, while the three dimensional structure of Allophycocyanin purified from Gch was determined by X-ray diffraction method and the biological unit was determined as a trimer by size exclusion chromatography. The protein sequences of all the components of the core were obtained by sequencing the corresponding genes and their expression confirmed by transcriptomic analysis. These subunits have seldom been reported in red algae, but not in Gracilaria chilensis. The subunits not present in the crystallographic structure were modeled to build the different composition of trimers. This article proposes structural models for the different types of trimers present in the core of phycobilisomes of Gch as a first step towards the final model for energy transfer in this system.
Highlights
Aquatic organisms, such as cyanobacteria, eukaryotic red alga and cryptophyta, present accessory light harvesting protein complexes called phycobilisomes (PBS) [1]
The core of PBS from Gracilaria chilensis: The structure of different composition trimers the PBS in this algae has been focused on understanding how the organization, i.e. composition, structure or component interactions of the PBS provides the frame for the high efficiency of the energy transfer towards photosystem II (PSII)
This structure was used to model the rest of the subunits, which may form a functional Allophycocyanin, and thereby to build the trimers proposed for the core in cyanobacteria, as our first approach to the analysis of the contribution of both the subunits in the core and Linker core (LC) to the energy transfer pathways in an eukaryotic red alga
Summary
Aquatic organisms, such as cyanobacteria, eukaryotic red alga and cryptophyta, present accessory light harvesting protein complexes called phycobilisomes (PBS) [1]. The core of PBS from Gracilaria chilensis: The structure of different composition trimers
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