Structure of AcrH–AopB Chaperone-Translocator Complex Reveals a Role for Membrane Hairpins in Type III Secretion System Translocon Assembly

Abstract

Type III secretion systems (T3SSs) are adopted by pathogenic bacteria for the transport of effector proteins into host cells through the translocon pore composed of major and minor translocator proteins. Both translocators require a dedicated chaperone for solubility. Despite tremendous efforts in the past, structural information regarding the chaperone-translocator complex and the topology of the translocon pore have remained elusive. Here, we report the crystal structure of the major translocator, AopB, from Aeromonas hydrophila AH-1 in complex with its chaperone, AcrH. Overall, the structure revealed unique interactions between the various interfaces of AopB and AcrH, with the N-terminal "molecular anchor" of AopB crossing into the "N-terminal arm" of AcrH. AopB adopts a novel fold, and its transmembrane regions form two pairs of helical hairpins. From these structural studies and associated cellular assays, we deduced the topology of the assembled T3SS translocon; both termini remain extracellular after membrane insertion.

NMR identification of the binding surfaces involved in theSalmonellaandShigellaType III secretion tip-translocon protein-protein interactions

“…9 Our results are also in agreement with a recently published topology of the assembled major translocon protein AopB (a homolog of SipB and IpaB) from Aeromonas hydrophila. 29 Nguyen et al . 29 show that both the N and C-termini of AopB are extracellularly exposed when the translocon is assembled and therefore the N-terminal ectodomain of the major translocon protein is available for interaction with the tip protein.…”

“…28 Further, Nguyen et al . 29 recently proposed a crystallography-derived model of the membrane-insertion and topology of the SipB/IpaB counterpart in Aeromonas hydrophila , AopB.…”

The type III secretion system needle, tip, and translocon

“…These proteins are predicted to contain coiled‐coils and intrinsically disordered regions . Currently, the only known atomic structures of the translocon proteins are the crystal structures of the N‐terminal ectodomains of the major translocon proteins Shigella IpaB and Salmonella SipB, which form three helix bundles; and that of Aeromonas AopB in complex with its chaperone (Figure c). The structure of the translocon at atomic resolution is currently unknown.…”

“…(b) Domain maps of the minor and major translocon proteins. (c) Crystal structures of the major translocon proteins IpaB and SipB N‐terminal domains; and AopB…”

“…100 Currently, the only available atomic structure for a translocon protein that includes the transmembrane regions, is that of the crystal structure of Aeromonas hydrophilia major translocon protein AopB in complex with its chaperone AcrH (Figure 5c). 76 Residues 40-264 of the 347-residue AopB formed a complex with AcrH without requiring any membrane mimics. The protein-protein interactions of AcrH with the transmembrane helices of AopB allowed formation of the AopB-AcrH complex in solution.…”