Combined thermodynamic and time-resolved structural analysis of interactions between AP2 and biomimetic plasma membranes provides insights into clathrin-mediated endocytosis
Clathrin mediated endocytosis (CME) is the main mechanism for swift and selective uptake of proteins into eukaryotic cells. CME is initiated by recruitment to the plasma membrane (PM) of the adaptor protein AP2, which recognizes the PM-associated lipid PtdIns(4,5)P2, as well as the protein cargo to be internalized. Nonetheless, many aspects of this process remain unclear due to their in vivo complexity. Here, a thermodynamic and time-resolved structural analysis of AP2 binding to different biomimetic PM was undertaken under physiological conditions using a combination of neutron reflectometry, interfacial tensiometry and rheology, and atomic force microscopy. The resultant in vitro data replicated previous in vivo observations, as well as yielded biophysical insights into normal and aborted CME. The presence of cargo may not be pivotal for the "activating" conformational change of AP2. However, the presence of cargo extends AP2s residence time on the membrane surface, due to slower on- and off-rates, thereby tentatively giving sufficient time for CME to proceed fully. Moreover, upon interaction with AP2, phospholipid lateral diffusion decreases markedly, inducing a gel phase attributed to creating a percolated network involving AP2 on the membrane, which could potentially serve as a mechanism for modulating subsequent clathrin binding.