Interfacing with the Light Harvesting System

The need of PSI for an extended antenna system is a main driving force for structural changes through the rearrangement, addition, and loss of subunits. This driving force was strong enough to induce the few substantial modifications in amino acids sequence that are observed between PSI of cyanobacteria and plastids. These modifications occur in the solvent-exposed loops of PsaA and PsaB. Modifications of PsaB provide the ability to bind additional chlorophylls that probably enable energy transfer from a proximal LHC-I complex in eukaryotes, whereas, in cyanobacteria, this loop interfaces with the unique cyanobacterial subunit PsaX. Another modification in eukaryotic PSI is that the PsaB loop allows interfacing with a LHC-II complex [68] (see Chapters 3 and 6).

Connecting to external light-harvesting complexes is the likely function of PsaF and PsaJ, PsaK in eukaryotic PSI. PsaF is associated (not coordinated) with several chlorophylls and carotenoids and may help to funnel excitation from external antenna systems (isiA and phycobilisomes in cyanobacteria and LHC in eukaryotes) to the PSI core. In photosynthetic eukaryotes, PsaF contains a 25 amino acid insertion shown to interface with plastocyanin -83, 84] - Connecting the reaction center core with external antenna systems is likely to be a function of PsaJ (two chlorophylls in eukaryotic, three in cyanobacterial PSI), PsaK (two chlorophylls), and the unique plant subunit PsaG.

Sharing of light-harvesting systems is also accomplished by the PSI trimer formation that is unique to cyanobacteria. PsaL forms the trimerization domain in PSI, stabilized by PsaI. In addition, PsaM possesses a single chlorophyll and may have a role in energy transfer between trimers. The presumed function of PsaL, which binds three chlorophylls, is to form the trimer and facilitate excitation energy transfer between the monomers. Cyanobacteria and plastids have executed different strategies for extended PSI light-harvesting system. In cyanobacteria the PSI trimer provide a flexible way of coupling PSI to isiA and Pcb antennae, while the strategy for increasing light capture in photosynthetic eukaryotes relies on optimized interactions with LHC complexes The subunit PsaH that interfaces with additional LHC in eukaryotic PSI is located exactly at the position where the three trimers intersect in cyanobacteria, and so PSI timer formation in photosynthetic eukaryotes is not possible. Another subunit that is unique to eukaryotic PSI is PsaG, which may also have a function in interfacing PSI with LHC. PsaG probably arose through duplication of PsaK [85].

PsaX is unique to cyanobacteria. The existence of PsaX was confirmed from the crystal structure of cyanobacterial PSI [67]. PsaX binds a single molecule of chlorophyll and may mediate energy transfer from isiA. The evolutionary significance of PsaX is unclear.

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