Journal article

Template-assembled melittin: structural and functional characterization of a designed, synthetic channel-forming protein

Template-assembled proteins (TASPs) comprising 4 peptide blocks, each of either the natural melittin sequence (melittin-TASP) or of a truncated melittin sequence (amino acid 6-26, melittin6-26-TASP), C-terminally linked to a (linear or cyclic) 10-amino acid template were synthesized and characterized, structurally by CD, by fluorescence spectroscopy, and by monolayer expts., and functionally, by elec. conductance measurements on planar bilayers and release expts. on dye-loaded vesicles. Melittin-TASP and the truncated analog preferentially adopt a-helical structures in methanol (56% and 52%, resp.) as in lipid membranes. Unlike in methanol, the melittin-TASP self-aggregates in water. On an air-water interface, the differently sized mols. can be self-assembled and compressed to a compact structure with a mol. area of around 600 .ANG.2, compatible with a 4-helix bundle preferentially oriented perpendicular to the interface. The proteins reveal a strong affinity for lipid membranes. A partition coeff. of 1.5 * 109 M-1 was evaluated from changes of the Trp fluorescence spectra of the TASP in water and in the lipid bilayer. In planar lipid bilayers, TASP mols. are able to form defined ion channels, exhibiting a small single-channel conductance of 7 pS (in 1 M NaCl). With increasing protein concn. in the lipid bilayer, addnl., larger conductance states of up to 1 nS were obsd. These states are likely to be formed by aggregated TGASP structures as inferred from a strongly voltage-dependent channel activity on membranes of large area. In this respect, melittin-TASP reveals channel features on the native peptide, but with a considerably lower variation in the size of the channel states. Compared to the free peptide, template-assembled melittin has a much higher membrane activity: it is about 100 times more effective in channel formation and 20 times more effective in releasing dye mols. from lipid vesicles. This demonstrates that the lytic properties are not solely related to channel formation. [on SciFinder (R)]


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