Chen Zhiming, Shi Zheng, Baumgart Tobias
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania.
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania.
Biophys J. 2015 Jul 21;109(2):298-307. doi: 10.1016/j.bpj.2015.06.010.
I-BAR proteins are well-known actin-cytoskeleton adaptors and have been observed to be involved in the formation of plasma membrane protrusions (filopodia). I-BAR proteins contain an all-helical, crescent-shaped IRSp53-MIM domain (IMD) dimer that is believed to be able to couple with a membrane shape. This coupling could involve the sensing and even the generation of negative plasma membrane curvature. Indeed, the in vitro studies have shown that IMDs can induce inward tubulation of liposomes. While N-BAR domains, which generate positive membrane curvature, have received a considerable amount of attention from both theory and experiments, the mechanisms of curvature coupling through IMDs are comparatively less studied and understood. Here we used a membrane-shape stability assay developed recently in our lab to quantitatively characterize IMD-induced membrane-shape transitions. We determined a membrane-shape stability diagram for IMDs that reveals how membrane tension and protein density can comodulate the generation of IMD-induced membrane protrusions. From comparison to analytical theory, we determine three key parameters that characterize the curvature coupling of IMD. We find that the curvature generation capacity of IMDs is significantly stronger compared to that of endophilin, an N-BAR protein known to be involved in plasma membrane shape transitions. Contrary to N-BAR domains, where amphipathic helix insertion is known to promote its membrane curvature generation, for IMDs we find that amphipathic helices inhibit membrane shape transitions, consistent with the inverse curvature that IMDs generate. Importantly, in both of these types of BAR domains, electrostatic interactions affect membrane-binding capacity, but do not appear to affect the curvature generation capacity of the protein. These two types of BAR domain proteins show qualitatively similar membrane shape stability diagrams, suggesting an underlying ubiquitous mechanism by which peripheral proteins regulate membrane curvature.
I-BAR蛋白是著名的肌动蛋白细胞骨架衔接蛋白，已观察到其参与质膜突起（丝状伪足）的形成。I-BAR蛋白包含一个全螺旋的新月形IRSp53-MIM结构域（IMD）二聚体，据信该二聚体能够与膜形状耦合。这种耦合可能涉及对负质膜曲率的感知甚至产生。实际上，体外研究表明IMD能够诱导脂质体向内成管。虽然产生正膜曲率的N-BAR结构域已受到理论和实验的大量关注，但通过IMD进行曲率耦合的机制相对较少被研究和理解。在这里，我们使用了我们实验室最近开发的膜形状稳定性测定法来定量表征IMD诱导的膜形状转变。我们确定了IMD的膜形状稳定性图，该图揭示了膜张力和蛋白质密度如何共同调节IMD诱导的膜突起的产生。通过与分析理论比较，我们确定了表征IMD曲率耦合的三个关键参数。我们发现，与已知参与质膜形状转变的N-BAR蛋白内吞蛋白相比，IMD的曲率产生能力明显更强。与已知两亲螺旋插入可促进其膜曲率产生的N-BAR结构域相反，对于IMD，我们发现两亲螺旋抑制膜形状转变，这与IMD产生的反向曲率一致。重要的是，在这两种类型的BAR结构域中，静电相互作用影响膜结合能力，但似乎不影响蛋白质的曲率产生能力。这两种类型的BAR结构域蛋白显示出定性相似的膜形状稳定性图，表明外周蛋白调节膜曲率的潜在普遍机制。
