Human atlastin-3 is a constitutive ER membrane fusion catalyst

Abstract

Homotypic membrane fusion catalyzed by the atlastin (ATL) GTPase sustains the branched endoplasmic reticulum (ER) network in metazoans. Our recent discovery that two of the three human ATL paralogs (ATL1/2) are C-terminally autoinhibited implied that relief of autoinhibition would be integral to the ATL fusion mechanism. An alternative hypothesis is that the third paralog ATL3 promotes constitutive ER fusion with relief of ATL1/2 autoinhibition used conditionally. However, published studies suggest ATL3 is a weak fusogen at best. Contrary to expectations, we demonstrate here that purified human ATL3 catalyzes efficient membrane fusion in vitro and is sufficient to sustain the ER network in triple knockout cells. Strikingly, ATL3 lacks any detectable C-terminal autoinhibition, like the invertebrate Drosophila ATL ortholog. Phylogenetic analysis of ATL C-termini indicates that C-terminal autoinhibition is a recent evolutionary innovation. We suggest that ATL3 is a constitutive ER fusion catalyst and that ATL1/2 autoinhibition likely evolved in vertebrates as a means of upregulating ER fusion activity on demand.

Dissecting the mechanism of atlastin-mediated homotypic membrane fusion at the single-molecule level

“…In eukaryotes, the endoplasmic reticulum (ER) spreads throughout the cell as a dynamic and continuous network of sheets and tubules with three-way junctions. This unique structure is formed by homotypic fusion between ER membranes 1 , 2 , which is catalyzed by dynamin-like integral membrane protein atlastin (ATL) in metazoans, Sey1p in yeast, and Root Hair Defective 3 (RHD3) in plants 3 – 12 . Because the ER network is critical to cellular function, the specific isoforms of ATLs are associated with a number of cellular processes, including the bone morphogenetic protein signaling pathway 13 – 16 , lipid droplet size regulation 17 , nuclear envelope assembly 18 , ER-phagy 19 , 20 , COPII formation 21 , autophagosome formation 22 , and flavivirus replication 23 , 24 .…”