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Immunity proteins and their specificity for endonuclease colicins: telling right from wrong in protein-protein
C Kleanthous1, A M Hemmings, G R Moore
1School of Biological Sciences, University of East Anglia, Norwich, UK. c.kleanthous@uea.ac.uk
Abstract:
Immunity proteins inhibit colicins, protein toxins released by bacteria during times of environmental stress, by binding and inactivating their cytotoxic domains. This protects the producing organism as it attempts to kill off competing bacteria. The cytotoxic domains of related colicins share a high degree of sequence identity, as do their corresponding immunity proteins, yet specificity and affinity are also high, with little non-cognate biological cross-protection evident under physiological conditions. We review recent work on DNase-specific immunity proteins, which shows that, although both cognate and non-cognate proteins can bind a single toxin, their affinities can differ by as much as 12 orders of magnitude. We have termed this mode of binding dual recognition, because the DNase-binding surface of an immunity protein is made up of two components, one conserved and the other variable. The strength of the binding interaction is dominated by the conserved residues, while neighbouring variable residues control specificity. Similar dual recognition systems may exist in other biological contexts, particularly where a protein must discriminate the right binding partner from numerous, structurally homologous alternatives.
Insights
Bacterial immunity proteins protect against toxic colicins by binding and inactivating them. A dual recognition mechanism, involving conserved and variable protein regions, dictates high-affinity binding and specificity.
Area of Science:
- Microbiology
- Molecular Biology
- Biochemistry
Background:
- Bacteria produce colicins, protein toxins that inhibit competing strains during stress.
- Immunity proteins neutralize these colicins by binding their cytotoxic domains, protecting the producing bacterium.
- Despite sequence similarities in colicins and immunity proteins, specific high-affinity interactions prevent cross-inhibition.
Purpose of the Study:
- To review recent findings on DNase-specific immunity proteins.
- To elucidate the mechanism of dual recognition in protein-toxin interactions.
- To explore the broader implications of dual recognition in biological systems.
Main Methods:
- Review of existing literature on DNase-specific immunity proteins and their cognate colicins.
- Analysis of binding affinities between cognate and non-cognate protein pairs.
- Examination of the structural components of the immunity protein's DNase-binding surface.
Main Results:
- Both cognate and non-cognate immunity proteins can bind the same toxin, but with vastly different affinities (up to 12 orders of magnitude).
- Dual recognition involves a conserved surface region dominating binding strength and a variable region conferring specificity.
- Conserved residues are key to the overall binding interaction, while variable residues fine-tune specificity.
Conclusions:
- Dual recognition is a key mechanism for specific and high-affinity binding between immunity proteins and colicins.
- This system allows discrimination between closely related toxins, preventing self-toxicity.
- Similar dual recognition principles may apply to other biological systems requiring discrimination of homologous partners.