Functions as an antenna for photosystem I (PSI) under iron-limiting conditions, when phycobilisomes disappear. Also functions as a dissipator of light energy, protecting cells from excessive light under iron-deficient conditions. Sequesters chlorophyll when cells are growing in iron-deficient conditions; it may bind up to 50% of the chlorophyll in iron-starved cells. Binds 16-17 chlorophyll a molecules per subunit. Under iron-starvation forms a complex with PSI trimers, where the trimer is surrounded by a ring composed of 18 IsiA subunits. Probably the major component of the CPVI-4 chlorophyll-protein complex. By iron stress, and also by exposure to oxidative stress such as hydrogen peroxide and methylviologen treatment. In cells lacking isiA under iron-deficient conditions, approximately 2-fold less chlorophyll is found per cell. Belongs to the PsbB/PsbC family. IsiA/Pcb subfamily.