Abstract

Originally discovered in the context of fatal neurodegenerative diseases, prions are proteins that can exist in two conformations: a normal one and a self-propagating form that can convert native proteins to the prion form. Surprisingly, prion proteins can also act as a stable, non-pathogenic epigenetic switch. The propagation of prion aggregates appears to be conserved across all domains of life, with extremely rare spontaneous transitions between the two conformations. The long timescales involved in prion conversion and loss have limited previous studies, leaving the underlying molecular mechanisms of these transitions still poorly understood. We set up a microfluidic platform that enabled tracking thousands of individual cells using quantitative time lapse fluorescence microscopy as they propagated and lost the prion state in Escherichia coli over hundreds of cell divisions. We focused on the recently discovered prion-forming domain in the single-stranded DNA binding protein (SSB) of the bacteria Campylobacter hominis, and observed two modes of prion propagation: small aggregates and an ultra-stable large aggregate located at the old pole of the cell. While cells harbouring the large aggregates maintained the prion state, the small aggregate sub-population lost the prion state following exponential decay. We further showed that, in this system, partitioning errors are the main cause of prion loss, and subsequently corroborated these results using orthologous prion domains. As many prion proteins have unknown function, we investigated the general physiological impact of the presence of prions in bacteria. We found that the prions imposed a small cost to the growth rate; however, our preliminary results suggest that they could also provide increased resistance to proteotoxic stresses. Our results emphasize the strengths of using a bacterial model system and our microfluidic setup to study the molecular mechanisms of prion propagation.