Membrane permeability of hydrocarbon-cross-linked peptides

Schafmeister, Po, and Verdine (another study) introduced a method using a hydrocarbon linker (staple) to stabilize a peptide in a helical configuration. One intended goal of this scheme is to facilitate the delivery of peptide drugs into target cells. Here, we investigate whether stapled peptides are intrinsically membrane permeable, by performing a case study on a stapled 12-mer peptide named NYAD-1. We found that the native peptide CAI (an HIV-1 inhibitor) does not bind to lipid bilayers, however NYAD-1 indeed permeates through lipid bilayers even at low solution concentrations. To understand the reason for the membrane permeability, we investigated the physical properties of NYAD-1 as a function of bound peptide/lipid molar ratio P/L. We found that NYAD-1 spontaneously binds to a lipid bilayer. At low P/L, the peptide primarily binds on the polar-apolar interface with its helical axis parallel to the bilayer, which has the effect of stretching the membrane area and thinning the membrane. The membrane thinning reaches its maximum at P/L ∼1/15-1/12 in DOPC bilayers. Additional bound peptides have little thinning effect and their helical axes are normal to the plane of bilayers. Thus, the stapled peptide has a membrane interaction behavior similar to helical antimicrobial peptides, such as magainin and melittin. We emphasize that not all peptides that bind to lipid bilayers in the α-helical form behave this way.