Figure 4.

Dumbbell shapes result from hemifusion and are determined by osmotic pressure and line tension. (A) Confocal image of an eDICE GUV encapsulating another GUV (scale bar, 5 μm). Such nested GUVs were rare ($\sim$5%–10$\%$ of GUVs) but present at frequencies comparable with dumbbells. (B) Proposed mechanism for dumbbell formation. (i) One vesicle with radius $R_{\text{tot}}$ encapsulates another vesicle with radius $R_{\mathrm{v}}$. (ii) The inner GUV bursts and hemifuses with the membrane of the outer GUV. The shape of the resulting dumbbell is determined by membrane tension and line tension along the hemifusion line. (C) Proposed microscopic configuration of the dumbbell neck (gray rectangle in B), where one inner and one outer leaflet (dim lobe, top) join four concentric leaflets in the bright lobe (bottom). (D) FRAP measurement of the bright lobe of a dumbbell reveals a recovery of the normalized fluorescence intensity from $\sim$2 (pre-bleach, $t<0$) to $\sim$1 (post-bleach, $t>0$) within seconds. (E and F) The model predicted dumbbell shapes (relative lobe sizes, E, and neck diameters relative to the average lobe diameters, F) that quantitatively match the experimental data (G and H). Experimental and simulated data represent $N=85$ and $N={10}^4$ dumbbell GUVs, respectively. Lines display linear fits (see legend for fit equations).