Are flat clathrin coats spring-loaded to curve?
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.
Source
Nanodissected elastically loaded clathrin lattices relax to increased curvature
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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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Clathrin-coated pit
- A patch of plasma membrane covered by a lattice of clathrin that invaginates to form a vesicle during endocytosis.
- Triskelion
- The three-legged building block of clathrin lattices, with an intrinsic curved (pyramidal) shape.
- Bending rigidity
- The energy cost to bend a sheet or shell away from its preferred curvature, here expressed in units of kBT.
- Elastically loaded lattice
- A network forced away from its preferred shape so that it stores elastic energy that can be released to change shape.
- Nanodissection
- Using an AFM tip with increased force to cut individual molecular connections in a structure.
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Quiz yourself
Why are flat clathrin lattices described as elastically loaded?
Common questions
Why would a flat clathrin lattice store energy?
Each triskelion prefers a curved shape, so holding many of them in flat hexagons bends them away from that preference, like a stretched spring.
How do the authors know the energy was stored and not added by the tip?
The tip only cut connections; the new pits then curved spontaneously, which would be unfavourable unless the lattice already stored bending energy.
Why does membrane tension matter?
Tension resists pulling membrane out of the plane; the model predicts that above a threshold tension forming spherical vesicles becomes unfavourable, stalling endocytosis.
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