Theoretical model for the formation of caveolae and similar membrane invaginations - PubMed (original) (raw)

Theoretical model for the formation of caveolae and similar membrane invaginations

Pierre Sens et al. Biophys J. 2004 Apr.

Abstract

We study a physical model for the formation of bud-like invaginations on fluid lipid membranes under tension, and apply this model to caveolae formation. We demonstrate that budding can be driven by membrane-bound proteins, provided that they exert asymmetric forces on the membrane that give rise to bending moments. In particular, caveolae formation does not necessarily require forces to be applied by the cytoskeleton. Our theoretical model is able to explain several features observed experimentally in caveolae, where proteins in the caveolin family are known to play a crucial role in the formation of caveolae buds. These include 1), the formation of caveolae buds with sizes in the 100-nm range and 2), that certain N- and C-termini deletion mutants result in vesicles that are an order-of-magnitude larger. Finally, we discuss the possible origin of the morphological striations that are observed on the surfaces of the caveolae.

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Figures

FIGURE 1

FIGURE 1

(a) Sketch of the blob model for the anchored protein aggregate and (b) force distribution for the two models used in the article: brush distribution (dashed, corresponding to a) and block distribution (solid). The membrane is pushed down by the corona of grafted polymers out to r = b = 5 nm and is pulled upwards by the anchored core inside _r_′ = a = 2 nm. (c) The corresponding membrane deformation u(r) in unit (formula image)a for _k_−1 = 30 nm. The brush distribution has a weaker effect on the membrane because the force is mostly concentrated near its center (r = 0). For aggregates residing on the cytoplasmic face of the membrane, including caveolin homo-oligomers, the cell interior would be above the membrane.

FIGURE 2

FIGURE 2

Sketch of bud formation upon increase of oligomer concentration. (a) Below the critical budding concentration (cbc), the membrane is uniformly covered by isolated oligomers. (b) At the cbc, buds have formed and outnumber isolated inclusions. (c) Above the cbc, the size and shape of the buds remains the same, and their number increases with the concentration.

FIGURE 3

FIGURE 3

Variation of the caveolae preferred radius R* (in nm) with the surface tension for a short-range attraction of _E_att = 0.5 k_B_T between brushlets (see Results for Various Force Distributions). Two values of the coupling strength _E_0 are displayed: _E_0 = 10 k_B_T (thin line) and _E_0 = 13 k_B_T (thick line). The inset shows the variation of the bud composition φ* (solid) and critical budding concentration (dashed) for the two coupling strengths. The other parameters are a = 2 nm, b = 5 nm, and κ = 20 k_B_T.

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