ADP ribosylation factor–like GTPase 6–interacting protein 5 (Arl6IP5) is an ER membrane-shaping protein that modulates ER-phagy

The endoplasmic reticulum (ER) is the membrane-bound organelle characterized by the reticular network of tubules. It is well established that the ER tubules are shaped by ER membrane proteins containing the conserved reticulon-homology domain (RHD). Membrane shaping by the RHD-containing proteins is also involved in the regulation of ER-phagy, selective autophagy of the ER. However, it remains unclear whether there exists ER membrane-shaping proteins other than the RHD-containing proteins. In this study, we characterize Arl6IP5, an ER membrane protein containing the conserved PRA1 domain, as an ER membrane-shaping protein. Upon overexpression, Arl6IP5 induces the extensive network of the ER tubules and constricts the ER membrane as judged by exclusion of a luminal ER enzyme from the ER tubules. The membrane constriction by Arl6IP5 allows the cells to maintain the tubular ER network in the absence of microtubules. siRNA-mediated knockdown of Arl6IP5 impairs the ER morphology, and the phenotype of the Arl6IP5 knockdown cells is rescued by exogenous expression of Arl6IP1, an RHD-containing protein. Furthermore, exogenous expression of Arl6IP5 rescues the phenotype of Arl6IP1 knockdown cells, and the PRA1 domain is sufficient to rescue it. Upon disruption of the possible short hairpin structures of the PRA1 domain, Arl6IP5 abolishes membrane constriction. The siRNA-mediated knockdown of Arl6IP5 impairs flux of the ER-phagy mediated by FAM134B. These results indicate that Arl6IP5 acts as an ER membrane-shaping protein involved in the regulation of ER-phagy, implying that the PRA1 domain may serve as a general membrane-shaping unit other than the RHD.