Lunapark ubiquitinates atlastin-2 for the tubular network formation of the endoplasmic reticulum

Endoplasmic reticulum (ER) tubules are interconnected by three-way junctions, resulting in the formation of a tubular ER network. Lunapark (Lnp) localizes to and stabilizes the three-way junctions. The N-terminal cytoplasmic domain in Lnp has a ubiquitin ligase activity. However, the molecular mechanism of how the ubiquitin ligase activity of Lnp is involved in the formation of the tubular ER network remains unknown. In this study, we examined whether the ER membrane proteins responsible for the formation of the tubular ER network are ubiquitinated by Lnp. We found that atlastin-2 (ATL2), an isoform of the ATL family mediating the generation of the three-way junctions by connecting the ER tubules, is a novel substrate for ubiquitination by Lnp. The localization of Lnp at the three-way junctions is important for ubiquitination of ATL2. Lysine 56, 57, 282 and 302 are the potential ubiquitination sites by Lnp. Silencing ATL2 decreased the number of the three-way junctions, and the expression of the ATL2 mutant in which the lysine residues are substituted with arginine failed to rescue the decrease of the three-way junctions in the ATL2 knocked-down cells. These results suggest that Lnp ubiquitinates ATL2 at the three-way junctions for the proper tubular ER network formation.