Structural and functional studies of bacterial Tripartite α-pore forming toxins

Pore-forming toxins (PFTs) are vital in both Prokaryotes and Eukaryotes and make up 30% of all bacterial toxins identified to date. Members of the ClyA family of α-PFTs have been found with one (ClyA), two (e.g YaxAB, XaxAB, PaxAB) or three (Hbl, Nhe, and Ahl) protein components that make the active pore. Pore structures are known for the bipartite family members, but only soluble structures (NheA, Hbl-B and AhlC) of the tripartite toxins, (the most complex system within the family), have been solved. As such, there is a pressing need for more structural and biochemical data to fully understand tripartite pore activity and assembly. The structure determination of A. hydrophilia AhlB in both soluble and pore forms as well as soluble AhlC head mutants have shown that AhlB is able to form pores (the first pore structure from a tripartite α-PFT), and also allowed roles to be proposed for both the B and C components of the Ahl toxin. TEM studies of Ahl pores, together with detailed biochemical studies of the activity of the AhlA, AhlB and AhlC components, has shown that AhlB, AhlBC and AhlABC pores can form, and identified three features common to all ClyA family α-PFTs. Characterisation of a second ClyA family tripartite α-PFT, Smh from Serratia marcescens, has shown that structures of SmhA and SmhB are well conserved with AhlB and NheA allowing identification of a latch which must be broken for the soluble to pore conformational change to occur. Having determined structures for all three components of Gram negative tripartite α-PFTs, a structure-based model of a complete pore has been constructed. Finally, although the Smh proteins share high structural similarities with both the Ahl and Nhe systems, biochemical assays show assembly mechanisms have diverged within the family.