# Structural basis for the assembly and energy transfer between the cyanobacterial PSI core and the double-layered IsiA proteins

## TL;DR

This paper reveals the structural details of how cyanobacteria assemble light-harvesting complexes under iron stress to survive in low-iron environments.

## Contribution

The study presents high-resolution structures of two PSI-IsiA supercomplexes, elucidating their assembly and energy transfer mechanisms under iron limitation.

## Key findings

- The PSI3-IsiA43 complex forms a closed double-ring structure around a trimeric PSI core.
- The PSI1-IsiA13 complex features a double-layered arrangement of IsiA proteins on a monomeric PSI core.
- Atomic force microscopy confirms the presence of multiple PSI-IsiA complexes in native thylakoid membranes.

## Abstract

Iron-limitation is a common stress factor in natural environments. To survive under iron-starved conditions, cyanobacteria overexpress iron stress-induced protein A (IsiA), which is crucial for light-harvesting and photoprotection. Multiple IsiA proteins form a single- or double-layered architecture encircling the photosystem I (PSI) core, forming various PSI-IsiA supercomplexes. The assembly and energy transfer mechanisms of double-layered PSI-IsiA supercomplexes remain unelucidated. Here, we present high-resolution structures of two PSI-IsiA supercomplexes isolated from the cyanobacterium Thermosynechococcus elongatus BP-1 cultured under iron-starved conditions. The PSI3-IsiA43 complex contains a trimeric PSI core surrounded by 43 IsiA subunits assembled into a closed double-ring. The PSI1-IsiA13 complex contains 13 IsiA proteins arranged in a double-layered architecture attached to the monomeric PSI core. Atomic force microscopy demonstrates the presence and distribution of different PSI-IsiA complexes within native thylakoid membranes isolated from iron-starved cells. Our findings provide insights into the structural variability and adaptive mechanisms of PSI-IsiA complexes.

Iron limitation induces the expression of IsiA in cyanobacteria, which serves as a light-harvesting antenna for PSI. Here, authors present two PSI-IsiA structures, elucidating the assembly and functional dynamics of these complexes under iron stress.