MFN1 structures reveal nucleotide-triggered dimerization critical for mitochondrial fusion

Abstract

Mitochondria are double-membrane organelles with varying shapes influenced by metabolic conditions, developmental stage, and environmental stimuli1–4. Their dynamic morphology is realized through regulated and balanced fusion and fission processes5, 6. Fusion is crucial for the health and physiological functions of mitochondria, including complementation of damaged mitochondrial DNAs and maintenance of membrane potential6–8. Mitofusins (Mfns) are dynamin-related GTPases essential for mitochondrial fusion9, 10. They are embedded in the mitochondrial outer membrane and thought to fuse adjacent mitochondria via concerted oligomerization and GTP hydrolysis11–13. However, the molecular mechanisms behind this process remains elusive. Here we present crystal structures of engineered human Mfn1 containing the GTPase domain and a helical domain in different stages of GTP hydrolysis. The helical domain is composed of elements from widely dispersed sequence regions of Mfn1 and resembles the Neck of the bacterial dynamin-like protein. The structures reveal unique features of its catalytic machinery and explain how GTP binding induces conformational changes to promote G domain dimerization in the transition state. Disruption of G domain dimerization abolishes the fusogenic activity of Mfn1. Moreover, a conserved aspartate trigger was found in Mfn1 to affect mitochondrial elongation, likely through a GTP-loading-dependent domain rearrangement. Based on these results, we propose a mechanistic model for Mfn1-mediated mitochondrial tethering. Our study provides important insights in the molecular basis of mitochondrial fusion and mitofusin-related human neuromuscular disorders14.

Structure, function, and regulation of mitofusin‐2 in health and disease

“…Both the GTPase and HR2 domains are crucial for the fusion process as mutations in these domains either prevent GTPase activity and thereby eliminate fusogenic activity, or disrupt the dimeric antiparallel coiled-coil structure and abolish membrane tethering (Qi et al, 2016). Consistent with this, Cao et al (2017) observed that dimerization of the GTPase domains is mediated by a conserved GTPase domain interface across the nucleotide binding site, resulting in activation of GTPase activity. Preventing dimerization of the GTPase domain led to impaired fusogenic activity of Mfn1, suggesting a crucial role for the G-interface in mitochondrial fusion (Cao et al, 2017).…”

“…Consistent with this, Cao et al (2017) observed that dimerization of the GTPase domains is mediated by a conserved GTPase domain interface across the nucleotide binding site, resulting in activation of GTPase activity. Preventing dimerization of the GTPase domain led to impaired fusogenic activity of Mfn1, suggesting a crucial role for the G-interface in mitochondrial fusion (Cao et al, 2017).…”

“…Recently, two independent studies have shed novel insights into Mfn structure and mitochondrial tethering (Qi et al, 2016;Cao et al, 2017). Crystal structures were generated from Mfn1 constructs containing an internal deletion of the second helical bundle and predicted transmembrane (TM) regions (Qi et al, 2016;Cao et al, 2017). The proposed model for fusion involves rotation of HB1 (helix bundle 1 extending from the GTPase domain) upon GTP hydrolysis, which allows HB2 (helix bindle 2 extending from the C terminal) to bend and attach to the GTPase domain, thereby bringing opposing mitochondrial membranes together (Qi et al, 2016).…”

“…2B) (Chen et al, 2003;Koshiba et al, 2004). Recently, two independent studies have shed novel insights into Mfn structure and mitochondrial tethering (Qi et al, 2016;Cao et al, 2017). Crystal structures were generated from Mfn1 constructs containing an internal deletion of the second helical bundle and predicted transmembrane (TM) regions (Qi et al, 2016;Cao et al, 2017).…”

Exome sequencing reveals a novel MFN2 missense mutation in a Chinese family with Charcot‑Marie‑Tooth type 2A

“…The pathogenicity of several missense variants at the same location including that found in the participant family were respectively predicted using online tools PolyPhen-2 (), Scale-Invariant Feature Transform (SIFT; ), MutationTaster () and M-CAP (). The three-dimensional structures of normal and mutant mitofusin 2 were generated by homology modeling using SWISS-MODEL () (12). The structure of mitofusin 2 was examined with the crystal structure of the truncated mitofusin 1 structure (pdb 5GOM).…”