# Dynamics of the formation of flat clathrin lattices in response to growth factor stimulus

## TL;DR

This study uses simulations to understand how flat clathrin lattices form and change on cell membranes, especially in response to growth factor signals.

## Contribution

A particle-based model reveals dynamic mechanisms of flat clathrin lattice formation and how they are regulated by EGF stimulation.

## Key findings

- Flat clathrin lattices are highly dynamic, with fluctuating cluster number, size, and lifespan even without external signals.
- Adaptor protein 2 (AP-2) and clathrin interactions significantly influence FCL dynamics.
- Simulated changes in AP-2, clathrin binding rate, and diffusion coefficient reproduce experimentally observed FCL trends under EGF stimulation.

## Abstract

Clathrin assemblies on the cell membrane are critical for endocytosis and signal transduction in cells. Specifically, the Ω-shaped clathrin assembly functions as the coat of endocytic vesicles, while the flat clathrin assembly, also known as the flat clathrin lattice (FCL), serves as a signaling hub for various pathways. Multiple flat clathrin lattices exist on the cell membrane, and these lattices grow after epidermal growth factor stimulation (EGF) and then return to baseline. In this work, we used a particle-based model to simulate the assembly and disassembly of flat clathrin lattices to capture these dynamics. We found that the formation of flat clathrin lattices is highly dynamic, that is, cluster number, size and dwelling time often change even in the absence of any stimulus. Moreover, these key features are affected by adaptor protein 2 (AP-2) number, clathrin-clathrin binding rate, and clathrin diffusion coefficient. Specifically, an increase in AP-2 number leads to the transition from no cluster, short-lived multiple small clusters, to a long-lasting single giant cluster. An increased clathrin-clathrin binding rate or decreased clathrin diffusion coefficient both result in an increased cluster number, reduced cluster size, and shortened dwelling time. Furthermore, we also predicted that under EGF stimulation, simultaneous changes in the AP-2 number, the clathrin-clathrin binding rate, and the clathrin diffusion coefficient can reproduce the experimentally observed trend of FCLs: an increase in cluster number and size in the first 30 minutes, followed by a decrease after 30 minutes. These findings reveal kinetic mechanisms underlying the formation of multiple FCLs and how EGF regulates FCL dynamics.

Clathrin is a protein essential for cell processes like endocytosis and signaling. It can form two shapes: one that helps create vesicles, and another, the “flat clathrin lattice” (FCL), which is involved in cellular signaling. In this study, we simulated how FCLs assemble and disassemble on the cell membrane. We found that FCLs are highly dynamic, with their size, number, and lifespan fluctuating even without external signals. Key factors like adaptor protein 2 (AP-2) and clathrin-clathrin interactions influence these dynamics. Our model also predicted that, under EGF stimulation, simultaneous changes in the AP-2 number, the clathrin-clathrin binding rate, and the clathrin diffusion coefficient can reproduce the experimentally observed trend of FCLs. These findings provide insight into how cells regulate FCL formation and signaling in response to external stimuli.