Interaction of the N-terminus of sterol carrier protein 2 with membranes: role of membrane curvature

Although neither the physiological function nor the mechanism of action of sterol carrier protein 2 (SCP2) is yet completely clear, it is thought that SCP2 interacts with membranes to elicit its biological effects. The result s presented here show that the SCP2 N-terminus, composed of two amphipathic alpha-helices, interacted preferentially with highly curved but not lower- curvature membranes containing anionic phospholipid. CD spectra of SCP2 sho wed up to 1.2-fold increased alpha-helical content, on the interaction of S CP2 with small unilamellar vesicles (SW) (median radius 10-14 nm) but less with large unilamellar vesicles (LW) (median radius 52-60 nm). Although enh anced interaction with the SW membranes was due in part to the radius of cu rvature and to the greater exposure of acidic phospholipid in the outer lea flet of the bilayer, simply increasing the molar percentage of acidic phosp holipid in the LUV membranes had much less effect on SCP2 binding, A simila r preferential interaction was observed with highly curved SUV as opposed t o LUV for the SCP2 N-terminal peptide 1-32SCP2 as well as structurally modi fied peptides in the order 1-32SCP2 = 10-32SCP2 > 1-24SCP2 much greater tha n 1-E20-32SCP2. The CD results were confirmed with an independent filtratio n binding assay, which showed that SCP2 bound 5-fold more to SUV than LUV, whereas its N-terminal peptides bound up to ii-fold better in the order 1-3 2SCP2 = 10-32SCP2 > 1-24SCP2 > 1-E20-32SCP2. Finally, cholesterol potentiat ed the binding of SCP2 and N-terminal peptides to anionic-phospholipid-cont aining SUV but not LW. These findings were consistent with the SCP2 N-termi nus being a membrane-binding domain that was highly dependent on membrane s urface curvature as well as on lipid composition.