Structural models of the different trimers present in the core of phycobilisomes from Gracilaria chilensis based on crystal structures and sequences

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.

Phycobiliproteins: Structural aspects, functional characteristics, and biotechnological perspectives

“…Together with the above, the spectrum observed under denaturing conditions corresponds to that of the αβ heterodimer, which is not affected under the conditions tested [35] . When analyzing the contact surfaces and interactions that stabilize the trimeric structure of APC, an average area of 500 Å 2 with 8 hydrogen bonds and 2 contact salt bridges between each pair of heterodimers can be observed [38] .

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Structure of cyanobacterial phycobilisome core revealed by structural modeling and chemical cross-linking

“…Both GpPBS core and PpPBS core are characterized by two unique features: lack of one trimeric ApcA/B disc at the distal end of ApcD in each basal cylinder ( Fig. 1A ) and thus a decreased copy number (by comparing to a four-Apc-trimer cylinder) of ApcC (Apc C; formerly L C ), a small polypeptide linker protein that stabilizes and caps each end of the core cylinders ( 20 , 21 ). The top cylinder in both GpPBS core and PpPBS core is featured by two Apc trimers, instead of four.…”

Red Seaweed Pigments from a Biotechnological Perspective

“…Recent reviews (from 2016 onwards) pertaining to PE are yet scarce, but existent (and discussed together with other PBP), and they seem to be more focused on those obtained from microalgae [4,9,17,19,32,55,106]. These include pigment availability and utilization [9], PBP/PBS structural chemistry [1,27,32,107], production [1,17,19,55], purification [19], and applications [1,4,19,32,62,106]. Recently, Zhang et al [108] uncovered the entire PBS complex (to 3.5 Å) from the red seaweed Griffithsia pacifica, through Cryogenic Electron Microscopy (cryo-EM).…”

“…Research into APC seems to be yet quite scarce when compared with research targeted on the other PBP found in red seaweeds, and it seems to be more focused on understanding mechanisms of structure [107,141,142] and synthesis [33,143]. Nevertheless, regarding extraction methods, diethyl ether gave good results, but methanol and ethanol were deemed improper in APC extraction of Furcelaria lumbricalis [74].…”