Role of the amino latch of staphylococcal alpha‐hemolysin and leukocidin in pore formation

Abstract

Staphylococcal α-hemolysin (αHL) and leukocidin are β-barrel pore-forming toxins (β PFTs) that are secreted by the bacterium as water-soluble monomers. Upon binding to susceptible cells, αHL assembles via an inactive prepore to form a homo-heptameric pore while the Luk pore is an octamer formed by the co-assembly of four copies each of LukF and LukS. The N terminus of αHL has been thought to play a vital role in pore formation, since the deletion of two N-terminal residues has shown to be arrested the assembly at the prepore stage. In the present study, we have re-examined assembly of αHL and Luk with a comprehensive set of truncation mutants. Surprisingly, we found that the ability of αHL to form functional pores is substantial even after truncation of up to seventeen amino acids. We then discovered that the mutation Ser-217→Asn, caused the complete inactivation observed in previous truncation mutants. Therefore, the residue 217 must interact indirectly with the distant N-terminus during pore formation. In addition, we provide evidence that an intact N-terminus prevents the premature oligomerization of αHL monomers in solution. Similarly, LukF subunits missing up to nineteen or LukS subunits missing up to fourteen N-terminal amino acids are capable of producing stable, functional hetero-oligomers with their WT counterparts. The simultaneous truncation of both LukF and LukS subunits is also tolerated. The results suggest that while the N-termini of β PFTs may undergo reorganization during assembly, they are not critical for the formation of functional pores.