Structural and functional characterization of cyanobacterial phycobilisomes
Phycobilisomes are large protein complexes in cyanobacteria and red algae, which upon light irradiation, initiates the photosynthesis process. The phycobilisome (PBS) complex typically contains different subunits, and energy efficiently flows from high (phycocyanin, PC) to low (allophycocyanin, APC) energy absorbing subunits. PC and APC are also composed of two basic subunits – a and b, which assemble into monomers (αβ), trimers (αβ)3, hexamers (two trimers) and rods (4 trimers or more). The smallest PBS identified to date is that of Acaryochloris marina (A. marina), composed of a single rod. The crystal structure of A. marina phycocyanin (AmPC), the major component of the A. marina PBS, was previously  determined to be a heterodimer of two alpha and two beta isoforms, superimposed in the asymmetric unit of the PC monomer structure. Several AmPC types were isolated, among them a PC type exhibiting allophycocyanin (APC)-like characteristics, suggesting it can efficiently serve as the PBS terminal emitter, since no such component was identified for the A. marina. The lack of APC along with the novel PC types, suggest that the A. marina PBS may operate without APC, solely based on one multi-functional PC.
We focus on structure determination of the APC-like PC, using x-ray crystallography and cryoEM, as well as characterization by confocal microscopy and mass spectrometry, to reveal the effect of the isoform composition on the energy transfer.