Binding of oligoarginine to membrane lipids and heparan sulfate:: Structural and thermodynamic characterization of a cell-penetrating peptide

Cell-penetrating peptides (CPPs) comprise a group of arginine-rich oligopeptides that are able to deliver exogenous cargo into cells. A first step in the internalization of CPPs is their binding to the cell surface, a reaction likely to involve membrane phospholipids and/or heparan sulfate proteoglycans (HSPGs). The present work characterizes the interaction of R-9, one of the most efficient CPPs, with either heparan sulfate (HS) or lipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol (POPG). Isothermal titration calorimetry shows that R9 binds to HS with high affinity. Assuming that HS has n independent and equivalent binding sites for R-9, we find an association constant of 3.1 x 10(6) M-1 at 28 degreesC. At this temperature, the reaction enthalpy is DeltaH(pep)degrees = -5.5 kcal/mol and similar to7 R-9 molecules bind per HS chain, which is equivalent to similar to0.95 cationic/anionic charge ratio. AH' decreases in magnitude upon an increase in temperature, and the reaction becomes entropy-driven at higher temperatures (greater than or equal to 37 degreesC). The positive heat-capacity change entailed by this reaction (DeltaC(p)degrees = +167 cal mol(-1) K-1) indicates the loss of polar residues on R-9-HS binding, suggesting that hydrophobic forces play no major role on binding. Calorimetric analysis of the interaction of R-9 with POPC/POPG (75:25) vesicles reveals an association constant of 8.2 x 10(4) M-1 at 28 T. Using a surface partition equilibrium model to correct for electrostatic effects, we find an intrinsic partition constant of similar to900 M-1, a value that is also confirmed by electrophoretic mobility measurements. This corresponds to an electrostatic contribution of similar to33% to the total free energy of binding. Deuterium nuclear magnetic resonance (NMR) shows no change in the headgroup, conformation of POPC and POPG, suggesting that binding takes place at some distance from the plane of the polar groups. P-31 NMR indicates that the lipid bilayer remains intact upon R9 binding. The fact that R9 binds with greater affinity to HS than to anionic lipid vesicles makes the former molecule a more likely target in binding this CPP to the cell surface.