Abstract
Microcin E492 (Mcc) is a pore-forming bacteriotoxin. Mcc activity is inhibited at the stationary phase by formation of amyloid-like aggregates in the culture. Here we report that, in a similar manner as prions, Mcc naturally exists as two conformers: a β-sheet-rich, protease-resistant, aggregated, inactive form (Mccia), and a soluble, protease-sensitive, active form (Mcca). The exogenous addition of culture medium containing Mccia or purified in vitro-generated Mccia into the culture induces the rapid and efficient conversion of Mcca into Mccia, which is maintained indefinitely after passaging, changing the bacterial phenotype. Mccia prion-like activity is conformation-dependent and could be reduced by immunodepleting Mccia. Interestingly, an internal region of Mcc shares sequence similarity with the central domain of the prion protein, which is key to the formation of mammalian prions. A synthetic peptide spanning this sequence forms amyloid-like fibrils in vitro and is capable of inducing the conversion of Mcca into Mcciain vivo, suggesting that this region corresponds to the prion domain of Mcc. Our findings suggest that Mcc is the first prokaryotic protein with prion properties which harnesses prion-like transmission to regulate protein function, suggesting that propagation of biological information using a prion-based conformational switch is an evolutionary conserved mechanism.
Highlights
Phenomena have been shown for several other proteins in diverse organisms
Microcin E492 (Mcc) activity was determined by the critical dilution method (CDM), as described in “Materials and Methods” (Fig. 1B)
The loss of Mcc activity in the stationary phase was paralleled by the formation of an altered conformation of Mcc that resulted in an increase in protease resistance
Summary
Phenomena have been shown for several other proteins in diverse organisms. For example, the neuronal isoform of Aplysia cytoplasmic polyadenylation element binding protein (CPEB) has been shown to undergo prion-like conformational changes that help to stabilize long term memory. The human mitochondrial antiviral signaling (MAVS) protein has been described to use the prion-like mechanism to activate and propagate antiviral innate immune signals. The human mitochondrial antiviral signaling (MAVS) protein has been described to use the prion-like mechanism to activate and propagate antiviral innate immune signals19 These reports provide evidence that the prion-like self-replication of conformational changes in a protein is not always pathogenic; rather, such conformational changes may regulate protein function and provide a selectable advantage in extreme conditions. We show prion-like, self-propagation of conformational changes in the bacteriotoxin Mcc that modulate the biological activity of the protein. The loss of Mcc activity appears to be due to the conversion of soluble protein into amyloid-like fibrils, similar to those associated with many human diseases including Alzheimer’s, Parkinson’s, Diabetes type II, and prion diseases. Mcc aggregates formed in vitro or in vivo share typical characteristics with disease-associated protein aggregates, including β-sheet rich secondary structure, ultrastructural morphology of the aggregates as long unbranched amyloid fibrils, binding to amyloid specific dyes and partial resistance to proteolysis
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