Structure of a monomeric photosystem I core associated with iron-stress-induced-A proteins from Anabaena sp. PCC 7120

IsiAs are unique light-harvesting complexes in cyanobacteria. Here, authors solved the structure of a PSI monomer associated with six IsiAs from Anabaena by cryo-EM, which provide insights into molecular diversity and functions of different IsiAs. Iron-stress-induced-A proteins (IsiAs) are expressed in cyanobacteria under iron-deficient conditions. The cyanobacterium Anabaena sp. PCC 7120 has four isiA genes; however, their binding property and functional roles in PSI are still missing. We analyzed a cryo-electron microscopy structure of a PSI-IsiA supercomplex isolated from Anabaena grown under an iron-deficient condition. The PSI-IsiA structure contains six IsiA subunits associated with the PsaA side of a PSI core monomer. Three of the six IsiA subunits were identified as IsiA1 and IsiA2. The PSI-IsiA structure lacks a PsaL subunit; instead, a C-terminal domain of IsiA2 occupies the position of PsaL, which inhibits the oligomerization of PSI, leading to the formation of a PSI monomer. Furthermore, excitation-energy transfer from IsiAs to PSI appeared with a time constant of 55 ps. These findings provide insights into both the molecular assembly of the Anabaena IsiA family and the functional roles of IsiAs.

Automated summary

By using cryo-EM, the authors determined the structure of a PSI monomer associated with six IsiA proteins in Anabaena, providing insights into the diversity and functions of IsiAs. IsiAs are expressed in cyanobacteria under iron-deficient conditions, and the binding properties and functional roles of IsiAs in PSI were still unknown in Anabaena. The cryo-EM structure of a PSI-IsiA supercomplex isolated from iron-deficient Anabaena revealed the association of six IsiA subunits with a PSI core monomer, and the absence of a PsaL subunit due to the occupation of its position by the C-terminal domain of IsiA2.