Spectroscopic studies of the C-terminal secretion signal of the Serratia marcescens haem acquisition protein (HasA) in various membrane-mimetic environments

The structure of a peptide comprising the last 56 C-terminal residues of the Serratia marcescens haem acquisition protein (HasA) secreted by an ATP-binding cassette exporter was examined by H-1-NMR, circular dichroic and fluorescence spectroscopies. The peptide, which contains the secretion signal of HasA, is efficiently secreted by the HasA transporter. It is largely unstructured and flexible in aqueous buffer solution, but its helical content increases upon addition of trifluoroethanol, detergents and lipids. By circular dichroism, a stable helical conformation is observed between 20% and 70% (by vol.) trifluoroethanol. The H-1 NMR spectrum was analysed at these two trifluoroethanol concentrations; residues 7-15, 21-30 and 40-50 were shown to form relatively stable helices. In the presence of neutral detergent, alpha-helix is induced to a similar extent upon micelle formation; in this case, fluorescence data indicate that at least the N-terminus of the peptide interacts with the micelle. In the presence of negatively charged detergent, alpha-helix is induced before micelle formation and the N-terminus of the peptide seems not to be involved in this interaction. In the presence of negatively charged liposomes, the peptide interacts with the vesicle, again inducing a helical conformation. However, the helical content remains lower than upon addition of trifluoroethanol or neutral micelles. These results are compared to those previously obtained with the secretion signal of one of the Erwinia chrysanthemi metalloproteases which are transported efficiently by the HasA transporter. Both signals exhibit similar conformational features, despite their low sequence similarity.