Membrane Tension Dictates the Spatiotemporal Heterogeneity of Endocytic Clathrin Coat Dynamics in Cells

Abstract

Clathrin-mediated endocytosis is essential for the cellular uptake of receptors and nutrients. Although under investigation since decades, the exact sequence of structural and molecular events remains elusive. There are two basic models that have been suggested for the way it proceeds. In the constant curvature model, it is assumed that clathrin-coated pits grow with constant curvature, determined by the geometry of clathrin triskelia. In the constant area model, it is assumed that clathrin triskelia first assemble into flat hexagonal arrays that later invaginate at constant surface area. This second model implicitly assumes that during bending, some hexagons are converted into pentagons. Here, we integrate data sets from correlative electron and light microscopy and quantify the sequence of ultrastructural rearrangements of the clathrin coat during endocytosis in mammalian cells with the help of some simple mathematical growth laws. Our main assumption is that the clathrin domain can grow only over the boundary. In the case of flat arrays, this requires some balancing process to prevent a runaway process, which we assume to grow in proportion to the domain area. In the case of curved arrays, pit closure is sufficient to limit growth and thus an additional process is not required. Our analysis shows that clathrin-coated structures initially grow flat but start to acquire curvature when 70% of the final clathrin content is reached. We find that this transition correlates with a change in the ratio of clathrin to adaptor protein AP2, and that membrane tension suppresses this transition. Hence, our analysis suggests that elements of both suggested models are present and that mechanical and cellular factors will decide about the relative weights of growth versus curvature formation.

Constraints and frustration in the clathrin-dependent endocytosis pathway

“…Indeed, membrane tension is not homogeneous across the plasma membrane [25] and in migrating mesenchymal cells, tension is higher at the front than at the back of the cells [26][27][28]. Thus, it has been shown that CRPs have a longer lifespan at the front than at the back of the cell [29]. These observations may explain why clathrin-coated structures tend to accumulate at the front of migrating cells [30,31].…”

“…En effet, la tension de membrane n'est pas homogène sur l'ensemble de la membrane plasmique [25] et dans des cellules mésenchymateuses en migration, la tension est plus élevée à l'avant qu'à l'arrière des cellules [26][27][28]. Ainsi, il a été démontré que les PRCs ont une durée de vie plus longue à l'avant qu'à l'arrière de la cellule [29]. Ces observations pourraient expliquer pourquoi les structures recouvertes de clathrine ont tendance à s'accumuler à l'avant des cellules en migration [30,31].…”