Molecular Mechanisms For GTPase-mediated Homotypic Membrane Fusion

In eukaryotic cells, cell organelles like mitochondria and endoplasmic reticulum (ER) constantly divide and fuse to maintain proper functions. Homotypic fusion of the mitochondrial outer membranes requires the mitofusin (MFN) proteins, a family of dynamin-like GTPases. MFN is anchored in the membrane by a hairpin of transmembrane (TM) segments, exposing both the N-terminal GTPase domain and the C-terminal tail (CT) to the cytosol. This arrangement is very similar to that of the atlastin (ATL) GTPases, which mediate homotypic fusion of the ER membranes. Based on our understanding of how ATL mediate the membrane fusion, we engineered various MFN-ATL chimeras to gain mechanistic insights into MFN-mediated fusion. When the GTPase domain of MFN1was replaced with the N-terminal cytosolic domain of ATL1, mitochondria aggregate around the nuclear, maybe it is the prefusion status of tethering. Similarly, when the cytosolic domains of MFN1were anchored to the ER by the TM segments of ATL1, the ER morphology defects caused by the lack of yeast ER fusogen were restored in yeast cells. In addition, the TM segments of MFNl exhibited homotypic interactions, and the CT of MFN1was exchangeable with that of ATL1, but did not mediate anti-parallel tethering as proposed previously.Another dynamin family that mediate the homotypic membrane fusion of the endoplasmic reticulum is mediated by a class of atlastin (ATL) GTPases. Three closely related ATL family members, named ATL1, ATL2, and ATL3, are found in humans. These ATLs are expressed in a tissue-specific manner, but their roles in ER fusion are not clear. Here, we show that ATL2knockdown causes a more prominent unbranched ER phenotype than ATL3knockdown. In mammalian cells with depleted levels of all ATLs, ER morphology defects can be restored efficiently by the expression of human ATL1or ATL2, but not ATL3. The different capacity of the three ATLs to maintain the ER network is also observed in yeast cells and correlates with nucleotide-dependent dimerization and GTPase activity, with ATL1being the strongest and ATL3being the weakest. These results suggest that ER fusion may be fine tuned in different cells that express different ATLs.Taken together, we suggest that MFN is sufficient for mediating homotypic membrane fusion, utilizing conserved mechanisms similar to ATL, and differential fusogenic capacity among family members may be a common way of regulating organellar dynamics in different cells.