Wang Xinran, Berro Julien, Ma Rui
Department of Physics, Xiamen University, Xiamen 361005, China.
Fujian Provincial Key Lab for Soft Functional Materials Research, Xiamen University, 361005, China.
bioRxiv. 2024 Sep 23:2024.08.13.607731. doi: 10.1101/2024.08.13.607731.
During clathrin-mediated endocytosis, a patch of flat plasma membrane is internalized to form a vesicle. In mammalian cells, how the clathrin coat deforms the membrane into a vesicle remains unclear and two main hypotheses have been debated. The "constant area" hypothesis assumes that clathrin molecules initially form a flat lattice on the membrane and deform the membrane by changing its intrinsic curvature while keeping the coating area constant. The alternative "constant curvature" hypothesis assumes that the intrinsic curvature of the clathrin lattice remains constant during the formation of a vesicle while the surface area it covers increases. Previous experimental studies were unable to unambiguously determine which hypothesis is correct. In this paper, we show that these two hypotheses are only two extreme cases of a continuum of vesiculation pathways if we account for the free energies associated with clathrin assembly and curvature generation. By tracing the negative gradient of the free energy, we define vesiculation pathways in the phase space of the coating area and the intrinsic curvature of clathrin coat. Our results show that, overall, the differences in measurable membrane morphology between the different models are not as big as expected, and the main differences are most salient at the early stage of endocytosis. Furthermore, the best fitting pathway to experimental data is not compatible with the constant-curvature model and resembles a constant-area-like pathway where the coating area initially expands with minor changes in the intrinsic curvature, later followed by a dramatic increase in the intrinsic curvature and minor change in the coating area. Our results also suggest that experimental measurement of the tip radius and the projected area of the clathrin coat will be the key to distinguish between models.
在网格蛋白介导的内吞作用过程中，一片扁平的质膜被内化形成一个囊泡。在哺乳动物细胞中，网格蛋白衣被如何将膜变形为囊泡仍不清楚，两种主要假说一直存在争议。“恒定面积”假说认为，网格蛋白分子最初在膜上形成一个扁平晶格，并通过改变膜的固有曲率使其变形，同时保持包被面积不变。另一种“恒定曲率”假说则认为，在囊泡形成过程中，网格蛋白晶格的固有曲率保持不变，而其覆盖的表面积增加。以往的实验研究无法明确确定哪种假说是正确的。在本文中，我们表明，如果考虑与网格蛋白组装和曲率产生相关的自由能，这两种假说只是一系列囊泡化途径的两个极端情况。通过追踪自由能的负梯度，我们在包被面积和网格蛋白衣被固有曲率的相空间中定义了囊泡化途径。我们的结果表明，总体而言，不同模型之间可测量的膜形态差异并不像预期的那么大，主要差异在胞吞作用的早期最为显著。此外，与实验数据拟合最好的途径与恒定曲率模型不相符，类似于一种恒定面积样途径，即包被面积最初随着固有曲率的微小变化而扩大，随后固有曲率急剧增加，包被面积变化较小。我们的结果还表明，对网格蛋白衣被的尖端半径和投影面积进行实验测量将是区分模型的关键。
