CONFORMATIONAL BEHAVIOR OF ESCHERICHIA-COLI OMPA SIGNAL PEPTIDES IN MEMBRANE MIMETIC ENVIRONMENTS

Nuclear magnetic resonance and circular dichroism (CD) studies of isol ated peptides corresponding to WT and mutant OmpA signal sequences are reported; all of the peptides adopt substantial amounts of alpha-heli cal structure both in 1:1 (v/v) trifluoroethanol (TFE)/water and in so dium dodecyl sulfate (SDS) micelles. In TFE/water, the helix begins af ter the positively charged N-terminal residues and is most stable in t he hydrophobic core, which correlates with results obtained previously for other signal sequences. The helix is weaker between the hydrophob ic core and the C-terminus; such a break in the helix appears to be co mmon to other signal peptides studied previously and could be of funct ional importance. No clear correlation could be established between th e helicity of the peptides in TFE/water and their in vivo activities. All the peptides have a higher alpha-helix content in SDS than in TFE/ water, and there is a good correlation between helix content in SDS an d in vivo activity. Helicity in SDS for the functional peptides increa ses both at the N-terminus and in the hydrophobic core, and is driven by a strong association of the core with the hydrophobic chains of the detergent. The extension of the helix toward the N-terminus may be a result of neutralization of the N-terminal positive charges by the hea dgroups of the micelles, which removes unfavorable electrostatic inter actions with the helix dipole. All these comparisons were facilitated by the use of upfield shifts of Halpha protons in helical regions rela tive to random coil chemical shifts, which also yielded estimates of h elical content that correlated well with the CD results.