The Influence of Membrane Lipids in Staphylococcus aureus Gamma-Hemolysins Pore Formation

Abstract

The natural target of Staphylococcus aureus bicomponent gamma-hemolysins are leucocyte cell membranes. Because a proteinaceous receptor has not been found yet, we checked for the importance of the different membrane lipid compositions by measuring the activity of the toxin on several pure lipid model membranes. We investigated the effect of membrane thickness, fluidity, and presence of nonbilayer lipids and found that the toxin pore-forming ability increased in the presence of phosphocholines with short saturated acyl chains or with unsaturated chains even though not short. An increase of activity was also evident in the presence of cone-shaped lipids like phosphatidylethanolamine or diphytanoylphosphatidylcholine, whereas cylindrical lipids, like sphingomyelin, did not favor the activity. All these results suggest that gamma-hemolysins could bind to the bilayer only if the phosphatidylcholine (PC) head is freely accessible. This condition is satisfied by the concurrent presence of cholesterol and certain lipids, as highlighted by the so-called umbrella model (J. Huang and G. W. Feigenson, Biophys J 76:2142-2157, 1999). According to this model, cholesterol could help to a better exposition of PC head groups only if acyl chains are short or unsaturated. In fact, phosphatidylcholines with more than 13 carbon atoms acyl chains can cover cholesterol molecules; in this way, PC head groups pack tightly, rendering them inaccessible to the toxin, which thus shows a reduced pore-forming ability.

The Bicomponent Pore-Forming Leucocidins of Staphylococcus aureus

“…However, enrichment of certain lipids in raft-like microdomains or under artificial conditions may allow for an enhanced poreforming capacity in a manner that is independent of receptor recognition (191,258). In fact, synthetic vesicles composed of defined lipids can lead to HlgAB-mediated pore formation in the absence of a protein receptor or GM1 (242). The efficiency of pore formation in this synthetic model is increased in the presence of phosphocholine-containing lipids as well as phosphatidylethanolamine but not sphingomyelin (242).…”

“…The mechanisms by which LukED, or any other leucocidin, exerts its cellular and/or species specificity have gone largely unrecognized over the past century. Most leucocidins are known to preferentially recognize certain cellular membrane lipids, and this lipid recognition is believed to facilitate the preporeto-pore transition (61,99,178,180,242,243). However, it is difficult to ascertain how membrane lipid recognition could also lead to the precise targeting of well-defined cellular subsets.…”

“…Beyond this assessment, no additional studies have investigated the possibility that gamma-hemolysin targets a proteinaceous receptor to mediate host cell killing. In contrast, a number of studies have suggested that gamma-hemolysin recognizes and binds to the membranes of red blood cells through direct interactions with host lipids and glycolipid derivatives (150,187,242,254). As early as 1980, Noda and colleagues identified the ganglioside GM1 as an efficient inhibitor of HlgCB-dependent (referred to as LukSF or leucocidin here) killing of rabbit leukocytes in vitro (150).…”

“…In fact, synthetic vesicles composed of defined lipids can lead to HlgAB-mediated pore formation in the absence of a protein receptor or GM1 (242). The efficiency of pore formation in this synthetic model is increased in the presence of phosphocholine-containing lipids as well as phosphatidylethanolamine but not sphingomyelin (242). Phosphatidylcholine has been shown to bind specifically to the F component (HlgB) with a molar ratio of 1:1, like that of GM1 with HlgA and HlgC, and the addition of exogenous phosphatidylcholine prevents the lytic activity of HlgCB on rabbit leukocytes (150,243).…”

Crystal structure of the octameric pore of staphylococcal γ-hemolysin reveals the β-barrel pore formation mechanism by two components

“…After forming a stable prepore, the β-barrel pore is formed. Pore formation requires the binding of phosphatidylcholine (PC) head groups to a cleft in the LukF component surrounded by Trp177 and Arg198 (Trp176 and Arg197 of LukF-PV) (13,15,16). The crystal structures of the monomeric forms of bicomponent β-PFTs [i.e., LukF (15), LukF-PV (17), and LukS-PV (18)], have been determined.…”