Shemesh Tom, Luini Alberto, Malhotra Vivek, Burger Koert N J, Kozlov Michael M
Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel.
Biophys J. 2003 Dec;85(6):3813-27. doi: 10.1016/S0006-3495(03)74796-1.
Membrane transport within mammalian cells is mediated by small vesicular as well as large pleiomorphic transport carriers (TCs). A major step in the formation of TCs is the creation and subsequent narrowing of a membrane neck connecting the emerging carrier with the initial membrane. In the case of small vesicular TCs, neck formation may be directly induced by the coat proteins that cover the emerging vesicle. However, the mechanism underlying the creation and narrowing of a membrane neck in the generation of large TCs remains unknown. We present a theoretical model for neck formation based on the elastic model of membranes. Our calculations suggest a lipid-driven mechanism with a central role for diacylglycerol (DAG). The model is applied to a well-characterized in vitro system that reconstitutes TC formation from the Golgi complex, namely the pearling and fission of Golgi tubules induced by CtBP/BARS, a protein that catalyzes the conversion of lysophosphatidic acid into phosphatidic acid. In view of the importance of a PA-DAG cycle in the formation of Golgi TCs, we assume that the newly formed phosphatidic acid undergoes rapid dephosphorylation into DAG. DAG possesses a unique molecular shape characterized by an extremely large negative spontaneous curvature, and it redistributes rapidly between the membrane monolayers and along the membrane surface. Coupling between local membrane curvature and local lipid composition results, by mutual enhancement, in constrictions of the tubule into membrane necks, and a related inhomogeneous lateral partitioning of DAG. Our theoretical model predicts the exact dimensions of the constrictions observed in the pearling Golgi tubules. Moreover, the model is able to explain membrane neck formation by physiologically relevant mole fractions of DAG.
哺乳动物细胞内的膜运输由小泡状以及大的多形性运输载体（TCs）介导。TCs形成的一个主要步骤是形成连接新生载体与初始膜的膜颈，并随后使其变窄。对于小泡状TCs而言，颈的形成可能直接由覆盖新生小泡的包被蛋白诱导。然而，在大TCs生成过程中膜颈形成和变窄的潜在机制仍然未知。我们基于膜的弹性模型提出了一个颈形成的理论模型。我们的计算表明存在一种由二酰基甘油（DAG）起核心作用的脂质驱动机制。该模型应用于一个特征明确的体外系统，该系统可从高尔基体复合物重构TC形成，即由CtBP/BARS诱导的高尔基体小管的成珠和裂变，CtBP/BARS是一种催化溶血磷脂酸转化为磷脂酸的蛋白质。鉴于PA - DAG循环在高尔基体TCs形成中的重要性，我们假设新形成的磷脂酸迅速去磷酸化形成DAG。DAG具有独特的分子形状，其特征是具有极大的负自发曲率，并且它在膜单层之间以及沿膜表面迅速重新分布。局部膜曲率与局部脂质组成之间的耦合通过相互增强导致小管收缩形成膜颈，并导致DAG的相关不均匀横向分配。我们的理论模型预测了在成珠的高尔基体小管中观察到的收缩的确切尺寸。此外，该模型能够通过生理相关摩尔分数的DAG来解释膜颈的形成。
