Terasaki Spiral Ramps in the Rough Endoplasmic Reticulum

We present a model describing the morphology as well as the assembly of “Terasaki ramps,” the recently discovered helicoidal connections linking adjacent sheets of the rough endoplasmic reticulum (ER). The fundamental unit is a localized symmetric double-ramped “parking garage” formed by two separated gently pitched, approximately helicoidal, ramps of opposite chiralities. This geometry is stabilized by a short-range repulsive interaction between ramps associated with bending energy which opposes the long-range attraction associated with tension. The ramp inner boundaries are themselves stabilized by the condensation of membrane-shaping proteins along their length. A mechanism for parking garage self-assembly is proposed involving the nucleation of dipoles at the center of tubular three-way junctions within the smooth ER. Our predictions are compared with the experimental data.

Figure 1

Symmetric minimal ramp compositions connecting parallel planar floors: floor-to-floor height $2\pi p\approx 500\mathrm{nm}$; interramp distance $R\approx 1\mu \mathrm{m}$, inner radius $r_0\approx 70\mathrm{nm}$.

Figure 2

Ramp inner boundary. The center of this tube is described by a space curve $\mathbf{Y}(s)$ that forms the boundary of the neutral surface (gray) describing the sheet.

Figure 3

Spontaneous dipole formation by detubulation: (a) triangular patches of positive Gaussian curvature lie at the top (and bottom) of the junction;(b) a small area difference between the top and bottom bilayers will generate pitch.