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Abstract
Lectin-like bacteriotoxic proteins, identified in several plant-associated bacteria, are able to selectively kill closely related species, including several phytopathogens, such as Pseudomonas syringae and Xanthomonas species, but so far their mode of action remains unrevealed. The crystal structure of LlpABW, the prototype lectin-like bacteriocin from Pseudomonas putida, reveals an architecture of two monocot mannose-binding lectin (MMBL) domains and a C-terminal β-hairpin extension. The C-terminal MMBL domain (C-domain) adopts a fold very similar to MMBL domains from plant lectins and contains a binding site for mannose and oligomannosides. Mutational analysis indicates that an intact sugar-binding pocket in this domain is crucial for bactericidal activity. The N-terminal MMBL domain (N-domain) adopts the same fold but is structurally more divergent and lacks a functional mannose-binding site. Differential activity of engineered N/C-domain chimers derived from two LlpA homologues with different killing spectra, disclosed that the N-domain determines target specificity. Apparently this bacteriocin is assembled from two structurally similar domains that evolved separately towards dedicated functions in target recognition and bacteriotoxicity.
Highlights
In most natural settings, complex interactions occur among microorganisms, ranging from nutritional co-operation to warfare among competitors
Microorganisms compete for space and nutrients, and a major strategy to assist in niche colonization is the deployment of antagonistic compounds directed at competitors, such as secondary metabolites and antibacterial peptides or proteins
For the protein containing two folded domains, we constructed site-specific mutants affected in carbohydrate binding and domain chimers from LlpA homologues to show that mannosespecific sugar binding mediated by one domain is required for activity and that the other domain determines target strain specificity
Summary
Complex interactions occur among microorganisms, ranging from nutritional co-operation to warfare among competitors. While a huge variety of secondary metabolites is used to target phylogenetically-distant competitors, ribosome-synthesized peptides or proteins are typically active against close relatives. These protein toxins are collectively referred to as bacteriocins, and may either be released into the environment or transferred to the host via specialized contact-dependent delivery systems [7,8,9]. Bacteriocins are structurally and mechanistically very diverse This is reflected in the bacteriocinogenic potential of the genus Pseudomonas [10]. Their R- and F-type pyocins are multi-subunit protein complexes evolutionarily related to contractile tails of bacteriophages [11,12,13]. The Xanthomonas LlpA precursor is proteolytically processed by removal of a characteristic Type II secretion signal peptide, whereas such N-terminal sequence is lacking in Pseudomonas homologues, indicating that secretory routes may differ among LlpA producers
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