The ER often appears as a tubular network. Previous work showed its tubular shaping involves a class of integral membrane proteins, comprising the reticulons and a protein family that includes DP1 in mammals and Yop1p in yeast. To investigate the tubule-forming abilities of these proteins, the authors purified Yop1p or a reticulon from yeast cells, mixed each with lipids, and reconstituted artificial membranes. The resulting proteoliposomes had the shape of tubules with a constant diameter (∼15–17 nm). These tubules were narrower than normal ER tubules because they contain a higher concentration of tubule-inducing proteins.
“The reticulons and DP1/Yop1 were known to be required for ER tubule formation,” Rapoport says. “This study shows they are sufficient.” The structure of these proteins is not known yet, but they form hairpin bends in the membrane and have the propensity to form oligomers. These characteristics suggest that they may form tubules via two effects: external wedging to induce local curvature, and protein linking to form a scaffold that enforces a smooth bend. “The synthesis of the two is most efficient,” says Rapoport. “We think this is a paradigm for how membranes are shaped in general.”