Phycocyanin: one complex, two states, two functions

Solar energy captured by pigments embedded in light-harvesting complexes can be transferred to neighboring pigments, dissipated, or emitted as fluorescence. Only when it reaches a reaction center is the excitation energy stabilized in the form of a charge separation and converted into chemical energy. Well-directed and regulated energy transfer within the network of pigments is therefore of crucial importance for the success of the photosynthetic processes. Using single-molecule spectroscopy, we show that phycocyanin can dynamically switch between two spectrally distinct states originating from two different conformations. Unexpectedly, one of the two states has a red-shifted emission spectrum. This state is not involved in energy dissipation; instead, we propose that it is involved in direct energy transfer to photosystem I. Finally, our findings suggest that the function of linker proteins in phycobilisomes is to stabilize one state or the other, thus controlling the light-harvesting functions of phycocyanin.M.G. acknowledges the European Molecular Biology Organization for funding his Long Term Fellowship and Claude Leon Foundation for his Postdoctoral Fellowship. The work of M.G., T.P.J.K., J.M.G., and R.v.G. was supported from an advanced investigator grant (267333, PHOTPROT) to R.v.G. from the European Research Council and the TOP grant (700.58.305) from the Foundation of Chemical Sciences part of NWO. R.v.G. gratefully acknowledges his “Academy Professor” grant from the Royal Netherlands Academy of Arts and Sciences (KNAW). T.P.J.K. was further supported by the University of Pretoria’s Research Development Programme (Grant No. A0W679).http://pubs.acs.org/journal/jpclcd2019-03-05hj2019Physic