Lipid-Destabilizing Properties Of The Hydrophobic Helices H8 And H9 From Colicin E1

Colicins are toxic proteins produced by Escherichia coli that must cross the
membrane to exert their activity. The lipid insertion of their pf domain is
linked to a conformational change which enables the penetration of a hydrophobic
hairpin. They provide useful models to more generally study insertion of
proteins, channel formation and protein translocation in and across membranes. In
this paper, we study the lipid-destabilizing properties of helices H8 and H9
forming the hydrophobic hairpin of colicin E1. Modelling analysis suggests that
those fragments behave like tilted peptides. The latter are characterized by an
asymmetric distribution of their hydrophobic residues when helical. They are able
to interact with a hydrophobic/hydrophilic interface (such as a lipid membrane)
and to destabilize the organized system into which they insert. Fluorescence
techniques using labelled liposomes clearly show that H9, and H8 to a lesser
extent, destabilize lipid particles, by inducing fusion and leakage. AFM assays
clearly indicate that H8 and especially H9 induce membrane fragilization. Holes
in the membrane are even observed in the presence of H9. This behaviour is close
to what is seen with viral fusion peptides. Those results suggest that the
peptides could be involved in the toroidal pore formation of colicin E1, notably
by disturbing the lipids and facilitating the insertion of the other, more
hydrophilic, helices that will form the pore. Since tilted, lipid-destabilizing
fragments are also common to membrane proteins and to signal sequences, we
suggest that tilted peptides should have an ubiquitous role in the mechanism of
insertion of proteins into membranes.