Homotypic fusion of ER membranes requires the dynamin-like GTPase Atlastin

Abstract

Establishment and maintenance of proper architecture is essential for endoplasmic reticulum (ER) function. Homotypic membrane fusion is required for ER biogenesis and maintenance, and has been shown to depend on GTP hydrolysis. Here we demonstrate that Drosophila Atlastin--the fly homologue of the mammalian GTPase atlastin 1 involved in hereditary spastic paraplegia--localizes on ER membranes and that its loss causes ER fragmentation. Drosophila Atlastin embedded in distinct membranes has the ability to form trans-oligomeric complexes and its overexpression induces enlargement of ER profiles, consistent with excessive fusion of ER membranes. In vitro experiments confirm that Atlastin autonomously drives membrane fusion in a GTP-dependent fashion. In contrast, GTPase-deficient Atlastin is inactive, unable to form trans-oligomeric complexes owing to failure to self-associate, and incapable of promoting fusion in vitro. These results demonstrate that Atlastin mediates membrane tethering and fusion and strongly suggest that it is the GTPase activity that is required for ER homotypic fusion.

Endoplasmic Reticulum Structure and Interconnections with Other Organelles

“…Depletion of endogenous Homo sapiens atl2 and atl3 by siRNA or overexpression of atlastin mutants in HeLa cells results in the formation of long, unbranched ER tubules, presumably as a result of the loss of three-way junctions (Hu et al 2009). In contrast, overexpression of wild-type atlastin in drosophila motor neurons leads to hyper fusion of ER membranes (Orso et al 2009) and an expansion of ER cisternae in mammalian tissue culture cells (Hu et al 2009). Proteoliposomes reconstituted with purified atlastin undergo membrane fusion in a GTPdependent manner, providing evidence that atlastins directly regulate ER fusion (Orso et al 2009).…”

“…Atlastin family members localize to the tubular ER and accumulate in a striking pattern at three-way junctions (Fig. 3C) (Rismanchi et al 2008;Hu et al 2009;Orso et al 2009;Park et al 2010b;Chen et al 2012). Atlastin and its yeast homolog Sey1 do not localize to the NE or to peripheral ER cisternae, suggesting that these proteins belong to a group of proteins that partition to regions of high membrane curvature (Fig.…”

“…Atlastin and its yeast homolog Sey1 do not localize to the NE or to peripheral ER cisternae, suggesting that these proteins belong to a group of proteins that partition to regions of high membrane curvature (Fig. 3C) (Rismanchi et al 2008;Hu et al 2009;Orso et al 2009;Park et al 2010b). Depletion of endogenous Homo sapiens atl2 and atl3 by siRNA or overexpression of atlastin mutants in HeLa cells results in the formation of long, unbranched ER tubules, presumably as a result of the loss of three-way junctions (Hu et al 2009).…”

“…The factors that regulate homotypic ER membrane fusion must also be ER-specific, this is important because the ER is closely apposed and potentially tethered to nearly every membranebound compartment in the cell. Homotypic ER fusion is regulated by the Atlastin family of dynamin-like GTPases (Rismanchi et al 2008;Hu et al 2009;Orso et al 2009;Anwar et al 2012). Atlastin family members localize to the tubular ER and accumulate in a striking pattern at three-way junctions (Fig.…”

ER-shaping proteins facilitate lipid exchange between the ER and mitochondria in S. cerevisiae

“…It has also been shown that the reticulon Rtn1p and Yop1p can directly tubulate membranes in vitro (Hu et al, 2008). Another family of proteins that play a role in maintaining ER shape are called atlastins; these are dynamin-like GTPases proteins that mediate ER-ER fusion Orso et al, 2009). Knock down of atlastins cause the formation of long unbranched ER tubules in mammalian cells.…”