Tzlil Shelly, Murray Diana, Ben-Shaul Avinoam
Department of Physical Chemistry and The Fritz Haber Research Center, Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Biophys J. 2008 Aug;95(4):1745-57. doi: 10.1529/biophysj.108.132522. Epub 2008 May 23.
The binding of the myristoylated alanine-rich C kinase substrate (MARCKS) to mixed, fluid, phospholipid membranes is modeled with a recently developed Monte Carlo simulation scheme. The central domain of MARCKS is both basic (zeta = +13) and hydrophobic (five Phe residues), and is flanked with two long chains, one ending with the myristoylated N-terminus. This natively unfolded protein is modeled as a flexible chain of "beads" representing the amino acid residues. The membranes contain neutral (zeta = 0), monovalent (zeta = -1), and tetravalent (zeta = -4) lipids, all of which are laterally mobile. MARCKS-membrane interaction is modeled by Debye-Hückel electrostatic potentials and semiempirical hydrophobic energies. In agreement with experiment, we find that membrane binding is mediated by electrostatic attraction of the basic domain to acidic lipids and membrane penetration of its hydrophobic moieties. The binding is opposed by configurational entropy losses and electrostatic membrane repulsion of the two long chains, and by lipid demixing upon adsorption. The simulations provide a physical model for how membrane-adsorbed MARCKS attracts several PIP(2) lipids (zeta = -4) to its vicinity, and how phosphorylation of the central domain (zeta = +13 to zeta = +7) triggers an "electrostatic switch", which weakens both the membrane interaction and PIP(2) sequestration. This scheme captures the essence of "discreteness of charge" at membrane surfaces and can examine the formation of membrane-mediated multicomponent macromolecular complexes that function in many cellular processes.
利用最近开发的蒙特卡罗模拟方案，对富含肉豆蔻酰化丙氨酸的蛋白激酶C底物（MARCKS）与混合的、流动性的磷脂膜的结合进行了建模。MARCKS的中央结构域兼具碱性（ζ = +13）和疏水性（五个苯丙氨酸残基），两侧有两条长链，其中一条以肉豆蔻酰化的N端结尾。这种天然未折叠的蛋白质被建模为代表氨基酸残基的“珠子”的柔性链。膜中含有中性（ζ = 0）、单价（ζ = -1）和四价（ζ = -4）脂质，所有这些脂质都能横向移动。MARCKS与膜的相互作用通过德拜-休克尔静电势和半经验疏水能量进行建模。与实验结果一致，我们发现膜结合是由碱性结构域与酸性脂质的静电吸引及其疏水部分的膜穿透介导的。结合受到两条长链的构象熵损失和静电膜排斥以及吸附时脂质的混合分离的阻碍。模拟为膜吸附的MARCKS如何将几种磷脂酰肌醇-4,5-二磷酸（PIP(2)）脂质（ζ = -4）吸引到其附近，以及中央结构域的磷酸化（ζ = +13变为ζ = +7）如何触发“静电开关”提供了一个物理模型，该“静电开关”会削弱膜相互作用和PIP(2)的隔离。该方案抓住了膜表面“电荷离散性”的本质，并可以研究在许多细胞过程中起作用的膜介导的多组分大分子复合物的形成。
