Perfringolysin O pore-forming complexes are predominantly integral multiples of six subunits

Perfringolysin O is a well-studied bacterial cytolysin that forms large oligomeric pores with a wide range of sizes on membranes via a prepore intermediate. Here we examined the sizes of both PFO prepore and pore complexes electrophoretically using a multi-stack-gradient gel and found that, unexpectedly, there are only at most seven predominant sizes of either pores or prepores. Complexes extracted from each band exhibit contour lengths that are integral multiples of six subunits. High-resolution atomic force microscopy images of PFO pore complexes in supported bilayers also reveal a predominant hexameric-based stoichiometry. Thus, these results reveal a previously unknown structural hierarchy in PFO complexes, with larger complexes apparently built up from hexameric sub-complexes. We suggest that different inter-subunit interactions within and between the hexamers result in a likewise difference in the coordination of the prepore-to-pore transition within and between the hexamers, and is thus a critical feature of the allostery of this large multi-subunit complex.