Helical allophycocyanin nanotubes absorb far-red light in a thermophilic cyanobacterium

To compete in certain low-light environments, some cyanobacteria express a paralog of the light-harvesting phycobiliprotein, allophycocyanin (AP), that strongly absorbs far-red light (FRL). Using cryo-electron microsco-py and time-resolved absorption spectroscopy, we reveal the structure-function relationship of this FRL-absorb-ing AP complex (FRL-AP) that is expressed during acclimation to low light and that likely associates with chlorophyll a-containing photosystem I. FRL-AP assembles as helical nanotubes rather than typical toroids due to alterations of the domain geometry within each subunit. Spectroscopic characterization suggests that FRL-AP nanotubes are somewhat inefficient antenna; however, the enhanced ability to harvest FRL when visible light is severely attenuated represents a beneficial trade-off. The results expand the known diversity of light -harvesting proteins in nature and exemplify how biological plasticity is achieved by balancing resource acces-sibility with efficiency.

Automated summary

To adapt to low-light environments, some cyanobacteria express a variant of the light-harvesting protein allophycocyanin (AP) that strongly absorbs far-red light (FRL). The structure-function relationship of this FRL-AP complex, expressed during acclimation to low light, was revealed using cryo-electron microscopy and time-resolved absorption spectroscopy. FRL-AP forms helical nanotubes instead of typical toroids due to altered domain geometry within each subunit. This study expands the understanding of light-harvesting proteins in nature and highlights the trade-off between resource accessibility and efficiency achieved through biological plasticity.