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NMR study of the preferred membrane orientation of polyisoprenols (dolichol) and the impact of their complex with polyisoprenyl recognition sequence peptides on membrane structure.
Earlier NMR studies showed that the polyisoprenols (PIs) dolichol (C95), dolichylphosphate (C95-P) and undecaprenylphosphate (C55-P) could alter membrane structure by inducing in the lamellar phospholipid (PL) bilayer a nonlamellar or hexagonal (Hex II) structure. The destabilizing effect of C95 and C95-P on host fatty acyl chains was supported by small angle X-ray diffraction and freeze-fracture electron microscopy. Our present 1H- and 31P-NMR studies show that the addition of a polyisoprenol recognition sequence (PIRS) peptide to nonlamellar membranes induced by the PIs can reverse the hexagonal structure phase back to a lamellar structure. This finding shows that the PI:PIRS docking complex can modulate the polymorphic phase transitions in PL membranes, a finding that may help us better understand how glycosyl carrier-linked sugar chains may traverse membranes. Using an energy-minimized molecular modeling approach, we also determined that the long axis of C95 in phosphatidylcholine (PC) membranes is oriented approximately parallel to the interface of the lipid bilayer, and that the head and tail groups are positioned near the bilayer interior. In contrast, the phosphate head group of C95-P is anchored at the PC bilayer, and the angle between the long axis of C95-P and the bilayer interface is about 758, giving rise to a preferred conformation more perpendicular to the plane of the bilayer. Molecular modeling calculations further revealed that up to five PIRS peptides can bind cooperatively to a single PI molecule, and this tethered structure has the potential to form a membrane channel. If such a channel were to exist in biological membranes, it could be of functional importance in glycoconjugate translocation, a finding that has not been previously reported.
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