Fonticins are phage tail-like bacteriocins produced by gram-negative bacterium Pragia fontium from the family Enterobacteriaceae. These high molecular weight bactericidal protein complexes can be divided to two different families; flexible ones (F-type) similar to Siphoviridae phage tails and contractile particles (R-type) evolutionally derived from Myoviridae phage tails.
Pragia fontium produces R-type particles that adsorb on the surface of sensitive bacterial cells and form large pores probably during the contraction in a way similar to Type VI Secretion System.
Pore-forming toxins, besides of their pathophysiological role, are a useful tool for characterisation of wide spectrum of macromolecules using single-molecule mass spectroscopy in solution. This method is based on quantification of changes in pore conductance influenced by the size of the analyte blocking the nano-pore.
Our goal was to use fonticin protein complexes for characterisation of bigger macromolecules and nanoparticles. The pore-forming activity of fonticins was characterised on Black Lipid Membranes; single channel conductance (G) is about 1.4 nS in 1 M NaCl. We tested blocking of fonticin pores by polyethylene glycol of various sizes showing, that the fonticin nano-pore is so wide that we can not detect individual blocations. Partitioning of these molecules to the pore can be detected as a distinct current noise accompanied by decrease of total conductance of the channel.
For the complete understanding of fonticins as a nano-device, it is important to identify its individual protein subunits and their exact structural organisation. For those purposes we have used SDS-electrophoresis, Mass Spectrometry and Transmission Electron Microscopy.
The rigid structure of fonticin nanotube in combination with constant conductance makes it a promising device for analysing the size and shape of nanoparticles and large macromolecules.