Demel R A, Van Doorn J M, Van der Horst D J
Centre for Biomembranes, University of Utrecht, The Netherlands.
Biochim Biophys Acta. 1992 Mar 4;1124(2):151-8. doi: 10.1016/0005-2760(92)90091-9.
In the formation of low-density lipophorin (LDLp) by the loading of diacylglycerol onto high-density lipophorin (HDLp) in insect hemolymph, apolipophorin III (apoLp-III) plays an essential role by binding to the increasing surface of the expanding lipoprotein particle. The present data on the surface properties of apoLp-III from Locusta migratoria demonstrate a preferential interaction with diacylglycerol. Injection of apoLp-III underneath a diacylglycerol monolayer results in a rapid interaction with the lipid; interaction with a phosphatidylcholine monolayer was considerably less. Locust apoLp-III binds with high affinity (Kd = 7.9.10(-9) M) to 1,2-diacylglycerol, which is consistent with its function in the LDLp particle; affinity for phosphatidylcholine is considerably lower. While the molecular area of locust apoLp-III in a monolayer is 2080 A2/molecule at the collapse pressure, in mixed monolayers of apoLp-III and lipid, the mean molecular area is decreased. Deglycosylation of the apoLp-III did not affect its interfacial stability. ApoLp-III from the moth Manduca sexta, which we included for comparison, demonstrated a similar reduction in molecular area resulting from interaction with lipid. These data do not support the hypothesis that interaction of apoLp-III with a lipid surface will lead to doubling of the molecular area of the protein (Kawooya, J.K., Meredith, S.C., Wells, M.A., Kézdy, F.J. and Law, J.H. (1986) J. Biol. Chem. 261, 13588-13591). The area of locust apoLp-III of 12.9 A2/amino acid residue at the collapse pressure is consistent with monolayers of alpha-helical proteins; circular dichroic spectra confirm a high alpha-helix content. The surface properties of apoLp-III reported here enable a high surface concentration of diacylglycerol in the LDLp particle, allowing the lipoprotein to act as an efficient reutilizable lipid shuttle.
在昆虫血淋巴中，通过将二酰甘油加载到高密度脂蛋白（HDLp）上形成低密度脂蛋白（LDLp）的过程中，载脂蛋白III（apoLp-III）通过与不断膨胀的脂蛋白颗粒增加的表面结合而发挥重要作用。目前关于飞蝗apoLp-III表面性质的数据表明它与二酰甘油存在优先相互作用。在二酰甘油单层下方注射apoLp-III会导致其与脂质迅速相互作用；而与磷脂酰胆碱单层的相互作用则要少得多。飞蝗apoLp-III以高亲和力（Kd = 7.9×10⁻⁹ M）与1,2-二酰甘油结合，这与其在LDLp颗粒中的功能一致；对磷脂酰胆碱的亲和力则低得多。虽然在塌陷压力下，飞蝗apoLp-III在单层中的分子面积为2080 Ų/分子，但在apoLp-III与脂质的混合单层中，平均分子面积会减小。apoLp-III的去糖基化不影响其界面稳定性。我们纳入用于比较的烟草天蛾的apoLp-III，也显示出与脂质相互作用导致分子面积有类似减小。这些数据不支持apoLp-III与脂质表面相互作用会导致蛋白质分子面积加倍的假说（Kawooya, J.K., Meredith, S.C., Wells, M.A., Kézdy, F.J. and Law, J.H. (1986) J. Biol. Chem. 261, 13588 - 13591）。在塌陷压力下，飞蝗apoLp-III的面积为12.9 Ų/氨基酸残基，这与α-螺旋蛋白的单层情况一致；圆二色光谱证实其α-螺旋含量很高。此处报道的apoLp-III的表面性质使得LDLp颗粒中能有高表面浓度的二酰甘油，从而使脂蛋白能够作为一种高效可再利用的脂质穿梭体发挥作用。
