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 results presented here show that the SCP2 N-terminus, composed of two amphipathic α-helices, interacted preferentially with highly curved but not lower-curvature membranes containing anionic phospholipid. CD spectra of SCP2 showed up to 1.2-fold increased α-helical content, on the interaction of SCP2 with small unilamellar vesicles (SUV) (median radius 10-14 nm) but less with large unilamellar vesicles (LUV) (median radius 52-60 nm). Although enhanced interaction with the SUV membranes was due in part to the radius of curvature and to the greater exposure of acidic phospholipid in the outer leaflet of the bilayer, simply increasing the molar percentage of acidic phospholipid in the LUV membranes had much less effect on SCP2 binding. A similar preferential interaction was observed with highly curved SUV as opposed to LUV for the SCP2 N-terminal peptide 1-32SCP2 as well as structurally modified peptides in the order 1-32SCP2 = 10-32SCP2 > 1-24SCP21-E20-32SCP2. The CD results were confirmed with an independent filtration binding assay, which showed that SCP2 bound 5-fold more to SUV than LUV, whereas its N-terminal peptides bound up to 4-fold better in the order 1-32SCP2 = 10-32SCP2 > 1-24SCP2 > 1-E20-32SCP2. Finally, cholesterol potentiated the binding of SCP2 and N-terminal peptides to anionic-phospholipid-containing SUV but not LUV. These findings were consistent with the SCP2 N-terminus being a membrane-binding domain that was highly dependent on membrane surface curvature as well as on lipid composition.

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