ER structure and function

Abstract

The ER forms a contiguous structure of interconnected sheets and tubules that spreads from the nuclear envelope to the cell cortex. Through its attachment to the cytoskeleton, the ER undergoes dynamic rearrangements, such as tubule extension and movement. ER shaping proteins (reticulons and DP1/Yop1p) play key roles in generating and maintaining the unique reticular morphology of the ER. Atlastin and its yeast homologue, Sey1p, mediate homotypic ER membrane fusion, which leads to the formation of new three-way junctions within the polygonal network. At these junctions, the Lunapark protein, Lnp1p, works in conjunction with the reticulons, DP1/Yop1p, and in antagonism to atlastin/Sey1p to maintain the network in a dynamic equilibrium. Defects in ER morphology have been linked to certain neurological disorders.

Figure 1

The ER network. (a) The ER network of COS-7 cells is labeled with mCherry-KDEL (red) and the junctions are labeled with Lnp1-GFP (green). Also marked: nucleus (N), ER cisternae and ER tubules. (b) The box area marked in the top panel is magnified below. Interconnected ER tubules form a network with three-way junctions marked by Lnp1-GFP.

Figure 2

Schematic diagram of the ER shaping proteins at three-way junctions. (a) Membrane topology around a three-way junction where reticulons (coral), DP1/Yop1p (coral), Lunapark (green and blue) and atlastin (purple and orange) insert into the outer leaflet of the phospholipid bilayer from the cytosolic side of the membrane. (b) A cross-section view of a three-way junction.