Plasticity in salt bridge allows fusion-competent ubiquitylation of mitofusins and Cdc48 recognition

Abstract

Mitochondrial fusion requires an alternating salt bridge between CMT2A-associated disease residues that enable GTP hydrolysis; Fzo1 ubiquitylation then licenses post-fusion recycling by Cdc48.

Mitochondrial Quality Control Governed by Ubiquitin

“…Under non-stressed conditions Fzo1 and additional other OMM proteins are targeted by another Cdc48 complex involving Npl4, Ufd1, and Doa1, also called Ufd3 (Wu et al, 2016) (Figure 1 and Table 1). This might however reflect protein quality control triggered by experimental tagging of the membrane-bound proteins, since non-tagged endogenous Fzo1 is rather stabilized by Cdc48, which exerts a regulatory role during OMM fusion (Simões et al, 2018;Anton et al, 2019). Turnover of mitofusins by MAD is widely conserved among species including Marf in flies (Ziviani et al, 2010;Wang et al, 2016) and Mfn1/2 in mammals (Tanaka et al, 2010;Chan et al, 2011;Xu et al, 2011).…”

Mitochondrial Surveillance by Cdc48/p97: MAD vs. Membrane Fusion

“…Interestingly, these regions map to mutations associated with CMT2A [204], suggesting pathogenic relevance. Their importance as a driving force for membrane merging could be confirmed with subsequent functional analysis, allowing further dissection of the fusion mechanism [75,76,194,195,[205][206][207][208][209][210][211][212]. Consistent with the self-assembly properties of DRPs, mitofusins can be found in multiple oligomerization states [184,207] (Figure 4C).…”

“…Furthermore, GTP hydrolysis leads to striking conformational changes in the G-domain itself, which allow Fzo1 ubiquitylation, Cdc48 recognition, and OMM fusion [76]. Prevention of Cdc48 binding to Fzo1 leads to the accumulation of Fzo1 at distinct foci at mitochondria, consistent with a role of Cdc48 in supporting membrane fusion by segregating Fzo1 complexes [75]. Also, in line with a segregation role of Cdc48, overexpression of Cdc48 resolves Fzo1 clusters that occur upon mutation of the small GTPase Arf1, a protein playing a critical role in membrane trafficking at mitochondria [173].…”

“…Other than its role in stress-induced MAD of mitofusins, Cdc48/p97 regulates mitochondrial dynamics by stimulating the activity of Fzo1, thus activating mitochondrial fusion [68,72,[74][75][76]132,133,182]. Generally, mitochondrial dynamics is mediated by large dynamin-like GTPase proteins (DRPs), a protein class whose function relies on self-oligomerization, coordinating GTP binding and hydrolysis with conformational changes [68,[183][184][185][186].…”

“…Under stress conditions, Cdc48/p97 extracts ubiquitylated mitofusin from OMM and hands it over for proteasomal degradation, which inhibits mitochondrial fusion and also regulates contact sites between mitochondria and the ER [68,73]. In contrast to MAD, Cdc48/p97 also stabilizes the yeast mitofusin Fzo1 and promotes mitochondrial fusion, e.g., by segregating Fzo1 clusters at fusion sites, thereby recycling it for a new fusion event [74][75][76]. In mammalian cells, further examples of p97-dependent MAD have been proposed, highlighting the complexity of the regulation of mitochondrial dynamics by Cdc48/p97.…”