Channel-Forming Abilities of Spontaneously Occurring α-Toxin Fragments from Staphylococcus aureus

Abstract

Pore formation by four spontaneously occurring alpha-toxin fragments from Staphylococcus aureus were investigated on liposome and erythrocyte membranes. All the isolated fragments bound to the different types of membranes and formed transmembrane channels in egg-phosphatidyl glycerol vesicles. Fragments of amino acids (aa) 9-293 (32 kD) and aa 13-293 (31 kD) formed heptamers, similar to the intact toxin, while the aa 72-293 (26 kD) fragment formed heptamers, octamers, and nonamers, as judged by gel electrophoresis of the liposomes. All isolated fragments induced release of chloride ions from large unilamellar vesicles. Channel formation was promoted by acidic pH and negatively charged lipid head groups. Also, the fragments' hemolytic activity was strongly decreased under neutral conditions but could be partially restored by acidification of the medium. We paid special attention to the 26-kD fragment, which, despite the loss of about one-fourth of the N-terminal part of alpha-toxin, did form transmembrane channels in liposomes. In light of the available data on channel formation by alpha-toxin, our results suggest that proteolytic degradation might be better tolerated than previously reported. Channel opening could be inhibited and open channels could be closed by zinc in the medium. Channel closure could be reversed by addition of EDTA. In contrast, digestion at the C terminus led to premature oligomerization and resulted in species with strongly diminished activity and dependent on protonation.