Endocytic proteins drive vesicle growth via instability in high membrane tension environment

Abstract

Clathrin-mediated endocytosis (CME) is a key pathway for transporting cargo into cells via membrane vesicles; it plays an integral role in nutrient import, signal transduction, neurotransmission, and cellular entry of pathogens and drug-carrying nanoparticles. Because CME entails substantial local remodeling of the plasma membrane, the presence of membrane tension offers resistance to bending and hence, vesicle formation. Experiments show that in such high-tension conditions, actin dynamics is required to carry out CME successfully. In this study, we build on these pioneering experimental studies to provide fundamental mechanistic insights into the roles of two key endocytic proteins-namely, actin and BAR proteins-in driving vesicle formation in high membrane tension environment. Our study reveals an actin force-induced "snapthrough instability" that triggers a rapid shape transition from a shallow invagination to a highly invaginated tubular structure. We show that the association of BAR proteins stabilizes vesicles and induces a milder instability. In addition, we present a rather counterintuitive role of BAR depolymerization in regulating the shape evolution of vesicles. We show that the dissociation of BAR proteins, supported by actin-BAR synergy, leads to considerable elongation and squeezing of vesicles. Going beyond the membrane geometry, we put forth a stress-based perspective for the onset of vesicle scission and predict the shapes and composition of detached vesicles. We present the snap-through transition and the high in-plane stress as possible explanations for the intriguing direct transformation of broad and shallow invaginations into detached vesicles in BAR mutant yeast cells.clathrin-mediated endocytosis | membrane tension | instability | actin | BAR proteins

Gaussian curvature directs the distribution of spontaneous curvature on bilayer membrane necks

“…The requirement for two distinct mechanisms in two regions of neck radii could be related to the energy barrier that we found as the neck radius of the catenoid decreases. Other evidence of switch-like behaviors are found in endocytosis where multiple studies have reported the existence of a snap-through instability 35,[64][65][66] . Furthermore, soap films, with no bending rigidity, also exhibit a structural instability when held as catenoids 45,67 .…”

Design principles for robust vesiculation in clathrin-mediated endocytosis

“…We use a modified version of the Helfrich energy that includes spatially varying spontaneous curvature C (θ α ) bending modulus κ(θ α ) and Gaussian modulus κG (θ α ) (42,48,50,52),…”

“…where ∆ is the surface Laplacian, p is the pressure difference across the membrane, λ is interpreted to be the membrane ten- sion, b αβ are components of the curvature tensor, f is a force per unit area applied to the membrane surface, and n is the unit normal to the surface (42,50). In this model, f represents the applied force exerted by the actin cytoskeleton; this force need not necessarily be normal to the membrane.…”

A stabilized finite element formulation for liquid shells and its application to lipid bilayers

“…The shape transitions occur as a result of lateral loading on the membranes from cytoskeletal filaments, such as actin, from osmotic pressure gradients across the membrane, or from reference configuration of finite element e membrane-protein interactions. For example, in endocytosis -a primary mode of transport of cargo between the exterior of the cell and its interior -proteins bind to the flat membrane and induce invaginations or bud shapes, followed by a tube formation by actin-mediated pulling forces [35,64].…”