Distinct Roles of Mic12 and Mic27 in the Mitochondrial Contact Site and Cristae Organizing System

Highlights

• Functional dissection of MICOS

• Mic27 promotes the stability of membrane-sculpting Mic10 oligomers.

• Mic12 is required for the coupling of MICOS subcomplexes.

• Mic12 and Mic27 differentially organize the MICOS complex.

Abstract

The mitochondrial inner membrane consists of two morphologically distinct domains, the inner boundary membrane and large invaginations termed cristae. Narrow membrane structures, the crista junctions, link these two domains. Maintenance of this elaborate architecture depends on the evolutionarily conserved mitochondrial contact site and cristae organizing system (MICOS), a multisubunit inner membrane protein complex. MICOS consists of two functional modules, a Mic60–Mic19 subcomplex that forms Mic60-mediated contact sites with the outer mitochondrial membrane and a Mic10–Mic12–Mic26–Mic27 membrane-sculpting subcomplex that contains large Mic10 oligomers. Deletion of MIC10 or MIC60 results in the loss of most crista junctions. Distinct views have been discussed about how the MICOS modules cooperate with each other. We searched for components required for the structural organization of MICOS and identified Mic12 and Mic27 as crucial factors with specific roles in MICOS complex formation. Mic27 promotes the stability of the Mic10 oligomers in the membrane-sculpting subcomplex, whereas Mic12 is required for the coupling of the two MICOS subcomplexes. We conclude that in addition to the MICOS core components Mic10 and Mic60, Mic12 and Mic27 play specific roles in the organization of the MICOS complex.

Introduction

Crista junctions are characteristic narrow, tubular, or slit-like membrane structures of the mitochondrial inner membrane. They connect the inner boundary membrane, the inner membrane domain that is directly adjacent to the outer membrane, to the cristae, the heterogeneously shaped membrane invaginations that protrude into the central mitochondrial matrix. The mitochondrial contact site and cristae organizing system (MICOS; also previously termed MINOS or MitOS) is a multimeric protein complex required for the maintenance of crista junctions [1], [2], [3], [4]. In the yeast Saccharomyces cerevisiae, the complex consists of six subunits: Mic10, Mic12, Mic19, Mic26, Mic27, and Mic60 (previously termed mitofilin/Fcj1) [5]. Except for the peripherally membrane-bound Mic19, all MICOS components are integral inner membrane proteins. MICOS has been conserved from fungi to humans, where it consists of MIC10, MIC19, MIC25 (a paralog of MIC19), MIC26, MIC27, MIC60, and the recently identified component QIL1 [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].

MICOS subunits are enriched at crista junctions [1], [13], [16], [17]. Loss of Mic10 or Mic60 leads to a dramatic loss of crista junctions, resulting in the detachment of cristae membranes from the inner boundary membrane and the formation of large internal membrane stacks [1], [2], [3], [16]. According to the severity of the mutant phenotypes, Mic10 and Mic60 have been termed the MICOS core components. As for the remaining non-core subunits, mutants lacking Mic12, Mic19, or Mic27 show a similar, yet less pronounced morphology phenotype, whereas the lack of Mic26, a paralog of Mic27, results in only minor alterations [1], [2], [3]. MICOS exhibits a modular organization: one subcomplex contains the core component Mic60 together with Mic19, and a second subcomplex is composed of the core component Mic10 together with Mic12, Mic26, and Mic27 [13], [18], [19]. Mic60 forms inner membrane—outer membrane contact sites through interaction with several resident outer membrane components: the protein translocase of the outer membrane (TOM complex); the sorting and assembly machinery (SAM/TOB complex); Ugo1, a component of the mitochondrial fusion machinery; and the abundant β-barrel protein porin [1], [2], [3], [7], [11], [20], [21], [22], [23]. Mic10 is a small, hydrophobic protein with two transmembrane domains that each contains a conserved glycine motif crucial for the formation of Mic10 oligomers. Mic10 oligomerization is required for the maintenance of crista junctions in vivo and Mic10 oligomers have the ability to deform membranes in vitro, indicating that multiple copies of Mic10 may form large membrane-sculpting scaffold complexes [18], [24]. Overexpression of either Mic10 or Mic60 leads to the formation of large amounts of crista junction-like structures [16], [18]. However, in each case, the inner membrane has a grossly aberrant appearance [16], [18], indicating that both Mic60-dependent contact site formation and Mic10-dependent membrane scaffolding have to be tightly coordinated for the formation of regular crista junctions and cristae membranes. It has been suggested that a key functional role of non-core MICOS components may be to mediate cooperation of the Mic60–Mic19 and the Mic10–Mic12–Mic26–Mic27 modules [13], [18], [19]. However, the exact molecular function of the non-core components in MICOS organization is unknown and different views have been proposed on how the two MICOS modules are connected [13], [18], [19].

In this study, we analyzed the role of non-core MICOS components in Mic10 oligomerization and in cooperation of the two MICOS subcomplexes. We find that Mic27 promotes Mic10 oligomerization, whereas Mic12 is required for connecting the Mic60–Mic19 and Mic10–Mic12–Mic26–Mic27 subcomplexes. The latter finding is reminiscent of the molecular function described for human QIL1 [13] and provides experimental support for a recent bioinformatics study that reported a remote homology between QIL1 and yeast Mic12 [25], suggesting that QIL1 may be the human Mic12 ortholog.