Elasticity and mechanical metamaterials
Are flat clathrin coats spring-loaded to curve?
Open access · cc by · source: Europe PMC
Flat clathrin lattices on cell membranes store elastic bending energy, and cutting their connections with an AFM tip makes them spontaneously curl into more curved pits.
Study at a glance
- Design
- Animal / in-vitro — High-speed AFM imaging and tip nanodissection of clathrin-coated pits on unroofed PTK2 cell membranes, plus AFM indentation of purified clathrin cages, interpreted with a continuum elastic energy model.
- N
- The text does not state the number of coated pits imaged or dissected; results are distributions over many pits on unroofed cells.
- Population
- Clathrin-coated pits on unroofed PTK2 (rat kangaroo kidney epithelial) cell membranes and purified calf-brain clathrin cages
- Outcome
- Pit surface area and radius of curvature, triskelion inter-arm angles, clathrin cage stiffness, and curvature change after lattice nanodissection
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Key findings
Pit shapes fitted neither the constant-area nor the constant-curvature model; area rose with radius of curvature in between the two predictions. Clathrin triskelia were strongly distorted, and more curved pits held more non-flat triskelia. Cage indentation gave a stiffness of about 0.08 N/m and a bending rigidity of roughly 373 kBT, far above the membrane's, and the energy model predicted a lowest-energy vesicle of radius about 55 nm, matching real vesicles in these cells. Every nanodissection of a flat lattice produced smaller pits with increased curvature, consistent with released stored elastic energy.
Methodology
The authors imaged clathrin-coated pits on unroofed cell membranes with high-speed atomic force microscopy, fitting spherical caps to get each pit's area and curvature and mapping the angles between clathrin arms. They measured the stiffness of purified clathrin cages to estimate the lattice's bending rigidity and built an energy model summing membrane bending, membrane tension, clathrin bending and clathrin polymerisation. They then used the AFM tip to cut individual clathrin connections in low-curvature lattices and watched how the shapes changed.
Limitations
Unroofed membranes are static snapshots, so the maturation pathway is inferred rather than watched in living cells. Real coated pits contain many other proteins, so the model attributes mechanics mainly to clathrin and may miss other contributors. The polymerisation energy was estimated indirectly as an upper bound, and several parameters (membrane tension, intrinsic clathrin radius) were assumed. The text does not report how many pits or dissections were analysed.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
A stiff, pre-strained scaffold can drive membrane curvature when released.
Protein coats can be far stiffer than the membrane and store bending energy: AFM indentation of clathrin cages gave a bending rigidity of about 373 kBT, and cutting connections in flat clathrin lattices on unroofed cells always produced smaller, more curved pits.
Evidence for the claim as stated.
A stiff, pre-strained scaffold can drive membrane curvature when released.
Protein coats can be far stiffer than the membrane and store bending energy: AFM indentation of clathrin cages gave a bending rigidity of about 373 kBT, and cutting connections in flat clathrin lattices on unroofed cells always produced smaller, more curved pits.
Scope note — Static unroofed membranes; number of pits not stated.
Limits the claim's scope: a different population, assay, or outcome.
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