The ER morphology-regulating lunapark protein induces the formation of stacked bilayer discs

Abstract

This is the final version of the article. Available from the publisher via the DOI in this recordLunapark (Lnp) is a conserved membrane protein that localizes to and stabilizes three-way junctions of the tubular ER network. In higher eukaryotes, phosphorylation of Lnp may contribute to the conversion of the ER from tubules to sheets during mitosis. Here, we report on the reconstitution of purified Lnp with phospholipids. Surprisingly, Lnp induces the formation of stacked membrane discs. Each disc is a bicelle, with Lnp sitting in the bilayer facing both directions. The interaction between bicelles is mediated by the cytosolic domains of Lnp, resulting in a constant distance between the discs. A phosphomimetic Lnp mutant shows reduced bicelle stacking. Based on these results, we propose that Lnp tethers ER membranes in vivo in a cell cycle–dependent manner. Lnp appears to be the first membrane protein that induces the formation of stacked bicelles.S Wang was supported by a fellowship from the Charles King Trust and RE Powers by a NIGMS T32 training grant (GM008313). We acknowledge the Max Planck Society and University of Exeter for supporting V Gold, in particular Werner Kühlbrandt and Deryck Mills at the Max Planck Institute of Biophysics. TA Rapoport is a Howard Hughes Medical Institute Investigator