Protein folding peptide membrane insertion

Lowik et demonstrated that the secondary structure of certain peptide sequences conjugated to single Ci8 alkyl chains at both the N- and C-termini could be induced upon incorporation into a liposome membrane. A sequence derived from the circumsporozoite (CS) protein of the malaria parasite Plasmodium falciparum was chosen, as within the natural protein the Asn-Pro-Asn-Ala repeat is known to adopt a j6-tum. Both the unmodified peptide in solution and the analogous peptide with only one alkyl chain showed random coil folding characteristics. However, when the double alkylated peptide was inserted into l,2-dimyristoyl-OT-glycero-3-phosphoethanol-amine (DSPC) liposomes the peptide folded into a P-hairpin. This simple approach is a convenient way for stabilizing a variety of peptides into their preferred secondary structure and might be employed in the presentation of multiple (hairpin) epitopes. 

In general, it is expected that most peptides composed of mixtures of polar and nonpolar residues adopt amphiphilic, interfacially active structures. Whether these structures are ordered or not depends on the sequence of amino acids. In contrast, nonpolar peptides are expected to partition from water to a nonpolar medium. Since they are disordered in the aqueous solution but, most likely, exist as a-helices in a nonpolar environment, they must fold before they reach thermodynamic equilibrium in this environment, presumably during the transfer across the interface. This is clearly relevant to the insertion and formation of transmembrane helices and the translocation of proteins across the membrane initiated by signal sequences. 

Recent research has suggested that short synthetic peptides containing different analogs of the first 19-23 amino acid residues of influenza hemaglutinin protein (HA) terminus may be attractive because of their pH-dependent lytic properties, with little activity at pH 7 but > 100-fold increase in transfection efficiency at pH 5. The lytic characteristics are revealed as the carboxyl groups of the aspartyl and glutamyl side chains are protonated, which allows the peptides to assume a a-helical conformation that can be inserted into the membrane bilayer. 

The two-stage model is conceptually very useful because it separates the thermodynamics of insertion from helix assembly. When examining the stability of membrane proteins in the bilayer, the second stage is presumably the most relevant since the extrusion of the protein back into the aqueous environment should be extremely unfavorable. This idea is supported by the experiments discussed below. The two-stage model may not contain sufficient detail to describe the folding process in all cases, however. For example, the F and G helices of bacteriorhodopsin do not spontaneously form helices in vesicles, indicating that these peptides require assistance from the remainder of the protein to fold properly (Hunt et al., 1997). 

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