Fluorescence energy transfer distance measurements using site-directed single cysteine mutants

The ion-channel-forming C-terminal fragment of colicin A binds to negatively charged lipid vesicles and provides an example of insertion of a soluble protein into a lipid bilayer. The soluble structure is known from X-ray crystallography and consists of a ten-helix bundle containing a hydrophobic helical hairpin. In this work fluorescence spectroscopy was used to study the membrane-bound structure. An extrinsic probe, N'-(iodoacetyl)-N'-(5-sulfo-l-naphthyl) ethylenediamine (IAEDANS) was attached to mutant proteins each of which bears a unique cysteine residue. Three mutants K39C (helix 2), T127C (between helices 6 and 7) and S16Crpt (helix 1, which bears a decapeptide repeat before the mutation) gave useful derivatives. In the soluble protein they showed emission wavelengths decreasing in the order K39C > T127C > S16Crpt and although all showed blue shifts on addition of dimyristoylphosphatidylglycerol (DMPG) this order was maintained in the membrane-bound state. These shifts were not indicative of deep membrane insertion. Polarization of IAEDANS revealed differences in mobility between mutants. The three tryptophan residues were used as a compound donor to IAEDANS in resonance energy transfer distance determinations. The values obtained for the soluble form were 1.2 A to 3.2 A longer than in the crystal structure. On addition of lipids the indicated distances increased: S16Crpt-I(AEDANS) 6.45 A (22%), K39C-I 5.45 A (18%) and T127C-I 2.4 A (14%). N-bromosuccinimide (NBS) completely abolishes the tryptophan emission from the thermolytic fragment. When lipids were added to a mixture containing ten NBS-treated channel-forming fragments to one IAEDANS labelled fragment the indicated distances increased rather more: S16Crpt-I 9.7 A (38%), K39C-I 8.1 A (36%) and T127C-I 2.5 A (16%). This showed that intermolecular transfer reduces the distance estimated in samples containing only labelled protein. The ensemble of results shows that the amphipathic helices of the C-terminal fragment open out on the surface of the lipid bilayer during the initial phase of membrane insertion.