Structural insights into crista junction formation by the Mic60-Mic19 complex

Abstract

Mitochondrial cristae membranes are the oxidative phosphorylation sites in cells. Crista junctions (CJs) form the highly curved neck regions of cristae and are thought to function as selective entry gates into the cristae space. Little is known about how CJs are generated and maintained. We show that the central coiled-coil domain of the mitochondrial contact and cristae organizing system (MICOS) subunit Mic60 forms an elongated, bow tie-shaped tetrameric assembly. Mic19 promotes Mic60 tetramerization via a conserved interface between the Mic60 mitofilin and Mic19 CHCH domains. Dimerization of mitofilin domains exposes a crescent-shaped membrane-binding site with convex curvature tailored to interact with curved CJ necks. Our study suggests that the Mic60-Mic19 subcomplex transverses CJs as a molecular strut, thereby controlling CJ architecture and function.

The development of intracytoplasmic membranes in alphaproteobacteria involves the conserved mitochondrial crista-developing protein Mic60

“…This was not unexpected as the HemD-Mic60 fusion protein is predicted to be membrane-anchored at its midpoint, having the HemD fragment exposed to the cytoplasm and Mic60 fragment on the periplasm. Moreover, the high degree of enrichment of Mic60 relative to BamA (Fig 5B, 5D) is consistent with the idea that this protein engages in homotypic interactions and forms homooligomers as its eukaryotic counterparts (Bock-Bierbaum et al 2022; Hessenberger et al 2017; John et al 2005).…”

“…Thus, it appears that both the formation of CSs and CJs/ICMJs have been conserved in alphaproteobacteria and mitochondria. The formation of mitochondrial CJs and CS by Mic60 are aided by Mic19 (Hessenberger et al 2017; Bock-Bierbaum et al 2022), but there is no evidence for a Mic19-like protein in alphaproteobacteria; our pull-downs did not reveal proteins with Coiled-Coil-Helix-Coiled-Coil-Helix (CHCH) motifs. It is plausible that CSs stabilize ICMs by providing an anchor to the OM at points of high membrane curvature, i.e., ICMJs (Tang et al 2020).…”

“…It is possible that BamA, through its interactions with Mic60, Orf52, OM Efflux, and RSP_7515, serves as the hub of a larger protein-interaction network, that, for example, facilitates lipid transfer and protein export to the outer membrane. Future studies are required to determine whether alphaproteobacterial Mic60 forms homodimers and homotetramers as recently reported for fungal Mic60 (Bock-Bierbaum et al 2022), and elucidate the molecular mechanisms by which alphaproteobacterial Mic60 interacts with BamA.…”

“…1A). Next, we predicted the tertiary structure of the Mic60 homologs of the yeast Lachancea thermotolerans , whose crystal structure was recently partially solved (Bock-Bierbaum et al 2022), and the alphaproteobacteria R. sphaeroides and R. palustris using AlphaFold2 (Fig. 1D-G).…”