Ribbon diagrams of the translocator domain of the classic autotransporter protein NalP (left) and the primary binding domain of Hia (right), a member of the recently recognized trimeric autotransporter subfamily. The helix inserted into the pore of NalP (left) is shown in red. Each subunit of the Hia adhesin (right) is colour-coded differently. Image courtesy of C. Oomen, Birkbeck College, London, UK.

Protein secretion from Gram-negative bacteria involves transport across both the inner and outer membranes. Now, two high-resolution structures in EMBO Journal shed light on the mechanisms of secretion by the autotransporter system and the role of these secreted proteins in virulence.

Autotransporters consist of three domains: an N-terminal signal peptide, which directs export of the precursor protein across the inner membrane before being cleaved; a C-terminal translocator, or β, domain, which inserts into the outer membrane; and a passenger domain, which is surface localized and is often released from the bacterium.

Piet Gros and colleagues have solved a high-resolution X-ray crystal structure of the translocator domain of NalP from Neisseria meningitidis. The structure reveals a 12-stranded β-barrel with a hydrophilic pore of 10 × 12.5 Å — a classical structure for outer-membrane proteins. But, unusually, the pore is filled with an N-terminal α-helix. Previous studies have proposed two distinct models for the protein-translocation process: transportation of the passenger domain through a pore formed by a single translocator domain; and transportation of the passenger domain through a pore formed by a multimer of translocator domains. The presence of hydrophobic residues on the exterior of the β-barrel seems inconsistent with the multimeric model; however, on the basis of the crystal structure the monomeric model would involve translocation of an unfolded protein. For these reasons, they propose an alternative mechanism that requires the Omp85 complex (which is required for the assembly of integral outer-membrane proteins).

In a separate study, Gabriel Waksman, Joseph St Geme and colleagues report the X-ray crystal structure of the HiaBD1 region of the passenger domain of the Haemophilus influenzae Hia protein — an adhesin that is primarily responsible for the binding of H. influenzae to host cell receptors. Unlike NalP, the passenger domain of Hia is not cleaved and remains attached to the translocator domain and, in contrast to the NalP structure, the HiaBD1 structure reveals a novel trimeric arrangement of individual HiaBD1 subunits, which has implications for the virulence of H. influenzae as it could enable multivalent interactions with the host cell. Sequence comparisons indicate that HiaBD1 is a member of a new subfamily of autotransporters in which both the passenger and translocator domains trimerize.

These studies provide clues as to the mechanisms and role of two autotransporters; however, further research is needed to determine not only if they do indeed represent two distinct classes of autotransporters, but also if the Omp85 complex is involved.