Elucidation of the crystal structure of the Listeria monocytogenes protein internalin B in complex with its tyrosine kinase Met receptor, and the creation of a new mouse model of listeriosis will greatly aid the understanding of Listeria.
Two papers recently published in Cell should provide new impetus to the Listeria research community. One presents the crystal structure of the Listeria monocytogenes protein internalin B (InlB) in complex with its tyrosine kinase Met receptor and the other describes the creation of a new mouse model of listeriosis.
L. monocytogenes enters non-phagocytic host cells using two cell-surface proteins, InlA and InlB, which bind to host cell receptors and induce the local cytoskeletal rearrangements that are necessary for bacterial uptake. InlA binds to E-cadherin, a major constituent of adherens junctions, to facilitate uptake into intestinal epithelial cells. Once this barrier has been breached, the bacterium can penetrate deeper tissues using the interaction between InlB and Met, which is found on the surface of many different cell types, including hepatocytes and endothelial cells.
The natural ligand for Met is hepatocyte growth factor (HGF). Previous work had indicated that InlB and HGF might have distinct binding sites on Met, and the first structural evidence to support this is now presented by Niemann, Heinz and colleagues in their crystal structure of the complex between the InlB Met-binding domain and a large fragment of the extracellular ectodomain of Met. The structure also contains a number of other interesting features. InlB binding to Met severely restricts the movement of the Met ectodomain, and the authors suggest that InlB acts as a 'molecular clamp' that effectively locks the receptor in a signalling-competent conformation. The authors propose a model in which, after the initial high-affinity binding and formation of the molecular clamp, InlB-induced activation of Met is completed by receptor clustering.
For many years Listeria researchers have faced a problem, as the uptake of L. monocytogenes by intestinal epithelial cells could not be modelled in mice because InlA does not bind to murine E-cadherin. One solution to this problem was the use of a transgenic mouse model that expresses human E-cadherin. Now, Wolf-Dieter Schubert, Andreas Lengeling and colleagues present an alternative solution — instead of humanizing a mouse they have 'murinized' the bacterium. Analysis of the complex between InlA and human E-cadherin revealed that this involves a surprisingly weak interaction. The authors were interested in whether this low binding affinity is biologically relevant, and used a rational protein-design approach to identify amino-acid substitutions at the interface between the two proteins that should increase the affinity of this interaction. They found that two single amino-acid substitutions (Ser192Asn and Tyr369Ser) in InlA increased the binding affinity by four orders of magnitude. Most interestingly, however, the binding specificity of InlA had been altered to allow it to bind to murine E-cadherin. Replacing the wild-type inlA gene with a gene encoding the doubly substituted protein revealed that the human infection can now be effectively recapitulated in mice.
Although L. monocytogenes is one of the most well-studied bacterial pathogens, there is still much to be learned about the interactions between this opportunistic intracellular pathogen and its hosts. These landmark papers should provide extremely useful directions for future research.
Sheilagh Molloy
References
Niemann, H. H. et al. Structure of the human receptor tyrosine kinase Met in complex with the Listeria invasion protein InlB. Cell130, 235–246 (2007) | Article | PubMed |
Wollert, T. et al. Extending the host range of Listeria monocytogenes by rational protein design. Cell129, 891–902 (2007) | Article | PubMed |
Hamon, M., Bierne, H. & Cossart, P. Listeria monocytogenes: a multifaceted model. Nature Rev. Microbiol.4, 423–434 (2006) | Article | PubMed |
