An extracellular hemolysin homolog from cyanobacterium Synechocystis sp. PCC6803

Abstract

In search of cyanobacterial extracellular Cu-chelator, we found that Synechocystis sp. PCC6803 secretes a homolog of hemolysin (toxin that lyses red blood cells of animals). The cyanobacterium was exposed to a Cu stress by culturing in BG-11 medium containing 3 µM Cu2+. When the culture medium was collected at the late log phase and subjected to hydroxyapatite chromatography, an elution peak of Cu was associated with a polypeptide of 173 kDa. The N-terminal amino acid sequence of the polypeptide was determined to be ALSPNVIAALQIMYTGRGVS. In a genome DNA database, this sequence had been registered as that of ¿hemolysin¿ with the N-terminal methionine. This fact indicated that there is no cleavable N-terminal signal peptide in the protein, as in hemolysin (HlyA) of Escherichia coli that has been shown to be secreted from the cells by an ABC transporter. The full amino acid sequence of the cyanobacterial hemolysin deduced from the DNA sequence indicated that the protein contains a number of b-roll motifs that are suggested to bind Ca2+ ions. It is conceivable that these sequences might be involved in the binding of Cu in our experiment. The fact that the autotroph produces hemolysin suggests that its physiological role is not the lysis of red blood cells. One possibility is that the cyanobacterial hemolysin serves to bind excess divalent cations in the environment to alleviate their stress.