Interaction of a small heat shock protein with light-harvesting cyanobacterial phycocyanins under stress conditions

Phycobiliproteins such as phycocyanins are the most abundant proteins found in cyanobacteria which are assembled to form the phycobilisome. Here, we showed that a small heat shock protein, HspA, interacts directly with phycocyanins from the cyanobacterium Synechococcus sp. strain PCC 7942 in vitro and suppresses inactivation of their light-harvesting functions due to heat denaturation in the presence of hydrogen peroxide. Under the denaturing conditions, phycobilisomes were de-assembled to lighter complexes and then aggregated. HspA associated with phycocyanins in the dissociated complexes, and suppressed the aggregation. The specific interaction between a small heat shock protein and phycocyanins was further supported by the fact that HspA and α-crystallin protected isolated phycocyanins from denaturation, while HtpG and lysozyme did not. The maximum protection was observed at a molar ratio of four HspA monomer per one phycocyanin (αβ) monomer.