SPG3A

210

367

The generation of the tubular network of the endoplasmic reticulum (ER) requires homotypic membrane fusion that is mediated by the dynamin-like, membrane-bound GTPase atlastin (ATL). Here, we have determined crystal structures of the cytosolic segment of human ATL1, which give insight into the mechanism of membrane fusion. The structures reveal a GTPase domain and athree-helix bundle, connected by a linker region. One structure corresponds to a prefusion state, in which ATL molecules in apposing membranes interact through their GTPase domains to form a dimer with the nucleotides bound at the interface. The other structure corresponds to a postfusion state generated after GTP hydrolysis and phosphate release. Compared with the prefusion structure, the three-helix bundles of the two ATL molecules undergo a major conformational change relative to the GTPase domains, which could pull the membranes together. The proposed fusion mechanism is supported by biochemical experiments and fusion assays with wild-type and mutant full-length Drosophila ATL. These experiments also show that membrane fusion is facilitated by the C-terminal cytosolic tails following the two transmembrane segments. Finally, our results show that mutations in ATL1 causing hereditary spastic paraplegia compromise homotypic ER fusion.protein structure | membrane remodeling | organelle shaping | spastic paraplegia gene 3A | endoplasmic reticulum network formation

Section: Resultssupporting

confidence: 58%

Structures of the atlastin GTPase provide insight into homotypic fusion of endoplasmic reticulum membranes

Bian

Klemm

Liu

et al. 2011

210

367

“…Also, to test whether the atlastin C-tail is necessary and sufficient for these phenomena, we created two additional constructs. Among the reported disease-causing mutations in atlastin is one that deletes most of the C-tail (21). This mutation, an insertion of an adenine at position 1688, changes the reading frame 10 aa into the C-tail, adds nonnative sequence, and introduces a premature stop codon.…”

Section: Resultsmentioning

confidence: 99%

Interaction of two hereditary spastic paraplegia gene products, spastin and atlastin, suggests a common pathway for axonal maintenance

Evans

Keller

Pavur

et al. 2006

108

107

Hereditary spastic paraplegia (HSP) is a neurodegenerative disorder that is characterized by retrograde axonal degeneration that primarily affects long spinal neurons. The disease is clinically heterogeneous, and there are >20 genetic loci identified. Here, we show a physical interaction between spastin and atlastin, two autosomal dominant HSP gene products. Spastin encodes a microtubule (MT)-severing AAA ATPase (ATPase associated with various activities), and atlastin encodes a Golgi-localized integral membrane protein GTPase. Atlastin does not regulate the enzymatic activity of spastin. We also identified a clinical mutation in atlastin outside of the GTPase domain that prevents interaction with spastin in cells. Therefore, we hypothesize that failure of appropriate interaction between these two HSP gene products may be pathogenetically relevant. These data indicate that at least a subset of HSP genes may define a cellular biological pathway that is important in axonal maintenance.microtubule ͉ AAA ATPase ͉ microtubule severing

“…Although the amphipathic helix of the CT is not absolutely essential for fusion, it is physiologically important, because ATL mutants lacking the CT cause HSP (29)(30)(31), and tailless human ATL and a point mutant in the C-terminal helix are defective in maintaining ER morphology in S. cerevisiae. Our results show that mutations in the helix dramatically reduce fusion activity and that the fusion defect of an ATL mutant lacking the C terminus is rescued when a synthetic peptide of the C-terminal amphipathic helix is added in trans.…”

Section: Discussionmentioning

confidence: 99%

Lipid interaction of the C terminus and association of the transmembrane segments facilitate atlastin-mediated homotypic endoplasmic reticulum fusion

Liu

Bian

Sun

et al. 2012

104

152

Generation of the tubular endoplasmic reticulum (ER) network requires homotypic membrane fusion. This is mediated in metazoans by atlastin (ATL), a dynamin‐like GTPase that consists of an N‐terminal cytosolic domain followed by two transmembrane segments (TMs) and a C‐terminal tail (CT). A GTP‐hydrolysis‐induced conformational change in the N‐terminal cytosolic domain is required for fusion, but it is unclear if this alone is sufficient for the fusion reaction. Here, we show using in vitro fusion assays that the CT and TMs are required for efficient fusion. A conserved amphipathic helix in the CT promotes fusion by interacting with and perturbing the lipid bilayer. The TMs not only serve as membrane anchors but also mediate ATL oligomerization. Point mutations in the CT or the TMs also impair ATL's ability to maintain ER membrane morphology in vivo. Our results suggest that protein‐lipid and protein‐protein interactions in the membrane cooperate with the conformational change of the cytosolic domain to achieve fusion and maintain the tubular ER network. These findings are relevant to mechanisms used by other membrane fusion proteins, such as mitofusins/Fzo1p, viral fusogens, and SNAREs.