Biological Relevance of Natural α-Toxin Fragments from Staphylococcus aureus

Abstract

Serine proteases represent an essential part of cellular homeostasis by generating biologically active peptides. In bacteria, proteolysis serves two different roles: a major housekeeping function and the destruction of foreign or target cell proteins, thereby promoting bacterial invasion. In the process, other virulence factors such as exotoxins become affected. In Staphylococcus aureus culture supernatant, the pore-forming alpha-toxin is cleaved by the coexpressed V8 protease and aureolysin. The oligomerizing and pore-forming abilities of five such spontaneously occurring N- and C-terminal alpha-toxin fragments were studied. (3)H-marked alpha-toxin fragments bound to rabbit erythrocyte membranes but only fragments with intact C termini, missing 8, 12 and 71 amino acids from their N-terminal, formed stable oligomers. All isolated fragments induced intoxication of mouse adrenocortical Y1 cells in vitro, though the nature of membrane damage for a fragment, degraded at its C terminus, remained obscure. Only one fragment, missing the first eight N-terminal amino acids, induced irreversible intoxication of Y1 cells in the same manner as the intact toxin. Four of the isolated fragments caused swelling, indicating altered channel formation. Fragments missing 12 and 71 amino acids from the N terminus occupied the same binding sites on Y1 cell membranes, though they inhibited membrane damage caused by intact toxin. In conclusion, N-terminal deletions up to 71 amino acids are tolerated, though the kinetics of channel formation and the channel's properties are altered. In contrast, digestion at the C terminus results in nonfunctional species.

Within-host evolution of bovine Staphylococcus aureus selects for a SigB-deficient pathotype characterized by reduced virulence but enhanced proteolytic activity and biofilm formation

“…In combination with a sigB -dependent expression of the fnbpA and clfA loci 22 , this could explain the diminished binding of the host-adapted, SigB-deficient isolate to immobilized fibronectin. High levels of secreted SspA and Aur have been reported to degrade Hla 37 , which possibly explains the reduced haemolytic activity on sheep-blood agar of the SigB-deficient, HA isolate, despite an elevated Hla expression and secretion 19 . Our data provide evidence that an enhanced proteolytic activity is advantageous in the course of a chronic infection, as it could possibly dampen bacterial immune recognition and toxicity to host cells, thus promoting access to new nutritional sources as well as escape from the host immune system, and might be responsible for the reduced virulence of the HA isolate in the intraperitoneal murine infection model.…”

Alpha-Toxin Promotes Staphylococcus aureus Mucosal Biofilm Formation

“…In the present study, tissue homogenates from CDC587 ( hla − ) contained small amounts of α-toxin and the protein appears to be slightly truncated compared to MNPE and purified α-toxin (Figure 3A). Several truncated forms of α-toxin have been reported in the literature (Kwak et al, 2010; Vecsey-Semjen et al, 2010). These fragments demonstrate significantly lower biological activity than the full-length protein, requiring much higher concentrations and longer incubation times to achieve similar pathological effects (Kwak et al, 2010).…”

“…Several truncated forms of α-toxin have been reported in the literature (Kwak et al, 2010; Vecsey-Semjen et al, 2010). These fragments demonstrate significantly lower biological activity than the full-length protein, requiring much higher concentrations and longer incubation times to achieve similar pathological effects (Kwak et al, 2010). Alpha-toxin is not detected from tissue homogenates infected with MN8 ( hla − ) despite of this strain being very similar to CDC587.…”

Prevalence of the SigB-Deficient Phenotype among Clinical Staphylococcus aureus Isolates Linked to Bovine Mastitis

“…The lower SigB activity may help counterbalance the high expression of agr /RNAIII and decrease the expression of α-hemolysin in the specific context of these isolates. Moreover, low SarA levels increase the expression and secretion of proteases that were shown to degrade secreted α-hemolysin, possibly contributing to the loss of the α-hemolysin phenotype [ 34 , 35 ]. Furthermore, we recently did not detect secretion of proteases but strong α-hemolysin secretion of a SigB-deficient strain under growth-limiting conditions, indicative of differential regulation of the two virulence factors downstream of the sigB operon [ 10 ].…”