Clathrin Assembly Regulated by Adaptor Proteins in Coarse-Grained Models

Abstract

The assembly of clathrin triskelia into polyhedral cages during endocytosis is regulated by adaptor proteins (APs). We explore how APs achieve this by developing coarse-grained models for clathrin and AP2, employing a Monte Carlo click interaction, to simulate their collective aggregation behavior. The phase diagrams indicate that a crucial role is played by the mechanical properties of the disordered linker segment of AP. We also present a statistical-mechanical theory for the assembly behavior of clathrin, yielding good agreement with our simulations and experimental data from the literature. Adaptor proteins are found to regulate the formation of clathrin coats under certain conditions, but can also suppress the formation of cages.

Early stages of clathrin aggregation at a membrane in coarse-grained simulations

“…59 In previous work, we explored the effect of sites located at the ankle and at the knee on cage assembly in solution, establishing that both possibilities permit AP-regulated clathrin aggregation. 51 Our study revealed that the mechanical properties of the AP linker region play a crucial role in regulating the assembly mechanism. Here, the β 2 bead binds to the clathrin ankle only, as this is the accepted site in the current literature.…”

“…In several preceding studies, we modelled clathrin as curved, rigid patchy particles to study the in vitro self-assembly of clathrin cages in bulk through Monte Carlo and Brownian dynamics simulations. [47][48][49][50][51] We here provide a brief description of the model and refer the reader to our previous publications for a more detailed discussion. The model triskelion consists of three identical legs stemming from a central "hub" (h) at a "pucker" angle χ relative to the normal vectorn h along the threefold rotational symmetry axis of the particle, see Fig.…”

“…In a previous study, 51 we introduced a coarse-grained model for adaptor proteins inspired by the AP2 complex and used it to study their role in the assembly process of clathrin triskelia in bulk through Monte Carlo simulations. Only those parts of the AP2 complex involved in clathrin binding were represented in the model.…”

“…[47][48][49][50] In a recent study, we developed a simulation model for AP2. 51 Several combinations of AP-binding locations on the clathrin leg were explored to separate combinations that enable cage assembly from those that do not, thereby arriving at the insight that the entropic spring linking the two clathrin motifs of AP2 is crucial to the functioning of this adaptor protein. The combination of both binding sites residing at the TD, which was not explicitly mentioned in that paper, did not prove able to promote cage assembly in the bulk.…”

“…Based on 10 6 triskelia in a cellular volume of 10 3 µm 3 , the clathrin concentration is estimated at 10 6 M. [53][54][55] A comparable concentration is used in the simulations, while the critical assembly concentration (CAC) of the model triskelion is about an order of magnitude higher. 49,51 The model for AP2 is equipped with two clathrin binding sites to enable the protein to bring two triskelia together and thereby induce cage assembly, as observed in in vitro experiments. 5 Our simulations of this process indicated that adaptor proteins-and, by implication, dimers of adaptor proteins [56][57][58] -are ineffectual when binding to terminal domains only because of the relatively large distance between TDs in a cage.…”