A minimal phycobilisome: Fusion and chromophorylation of the truncated core-membrane linker and phycocyanin

Phycobilisomes, the light-harvesting antennas in cyanobacteria and red algae, consist of an allophycocyanin core that is attached to the membrane via a core-membrane linker, and rods comprised of phycocyanin and often also phycoerythrin or phycoerythrocyanin. Phycobiliproteins show excellent energy transfer among the chromophores that renders them biomarkers with large Stokes-shifts absorbing over most of the visible spectrum and into the near infrared. Their application is limited, however, due to covalent binding of the chromophores and by solubility problems. We report construction of a water-soluble minimal chromophore-binding unit of the red-absorbing and fluorescing core-membrane linker. This was fused to minimal chromophore-binding units of phycocyanin. After double chromophorylation with phycocyanobilin, in E. coli, the fused phycobiliproteins absorbed light in the range of 610–660 nm, and fluoresced at ~ 670 nm, similar to phycobilisomes devoid of phycoerythr(ocyan)in. The fused phycobiliprotein could also be doubly chromophorylated with phycoerythrobilin, resulting in a chromoprotein absorbing around 540–575 nm, and fluorescing at ~ 585 nm. The broad absorptions and the large Stokes shifts render these chromoproteins candidates for imaging; they may also be helpful in studying phycobilisome assembly.

► We solubilize phycobilisome core-membrane linker by selective truncation.
► We fuse the truncated core-membrane linker with the minimal chromophore domain of phycocyanin.
► The fused constructs can be bi-chromophorylated with phycocyanobilin, phycoviolobilin, phycoerythrobilin and/or phycourobilin.
► The bi-chromophorylated constructs have broad absorptions, efficient energy transfer and large Stokes shifts.