Defining the interactome of the human mitochondrial ribosome identifies SMIM4 and TMEM223 as respiratory chain assembly factors

Abstract

Human mitochondria express a genome that encodes thirteen core subunits of the oxidative phosphorylation system (OXPHOS). These proteins insert into the inner membrane co-translationally. Therefore, mitochondrial ribosomes engage with the OXA1L-insertase and membrane-associated proteins, which support membrane insertion of translation products and early assembly steps into OXPHOS complexes. To identify ribosome-associated biogenesis factors for the OXPHOS system, we purified ribosomes and associated proteins from mitochondria. We identified TMEM223 as a ribosome-associated protein involved in complex IV biogenesis. TMEM223 stimulates the translation of COX1 mRNA and is a constituent of early COX1 assembly intermediates. Moreover, we show that SMIM4 together with C12ORF73 interacts with newly synthesized cytochrome b to support initial steps of complex III biogenesis in complex with UQCC1 and UQCC2. Our analyses define the interactome of the human mitochondrial ribosome and reveal novel assembly factors for complex III and IV biogenesis that link early assembly stages to the translation machinery.

Two neuronal peptides encoded from a single transcript regulate mitochondrial complex III inDrosophila

“…We propose a model where Sloth1 and Sloth2 peptides are translated from the same transcript, imported into mitochondria where they interact with each other and complex III to promote its assembly (Figure 10). Our results are supported by two recent studies published during preparation of this manuscript, in which human Sloth1 (SMIM4) and Sloth2 (C12orf73/Brawnin) were discovered as novel mitochondrial complex III assembly factors in cultured human cells and zebrafish (ZHANG et al 2020;DENNERLEIN et al 2021). Muti-cistronic genes are relatively rare in eukaryotes, but some have been characterized in Drosophila (GALINDO et al 2007;MAGNY et al 2013) and mammals (KARGINOV et al 2017).…”

“…SMIM4 and C12orf73 interact with complex III subunits UQCC1 and UQCRFS1, respectively (Z hang et al . 2020; D ennerlein et al . 2021).…”

“…Our results are supported by two recent studies published during preparation of this manuscript, in which human Sloth1 (SMIM4) and Sloth2 (C12orf73/Brawnin) were discovered as novel mitochondrial complex III assembly factors in cultured human cells and zebrafish (Z hang et al . 2020; D ennerlein et al . 2021).…”

“…It is also possible that these peptides form weak interactions with mitochondrial proteins, preventing their immunoprecipitation. Recently, human SMIM4 was identified in a proteomic screen (D ennerlein et al . 2021), human C12orf73 was identified in two proteomics screens (L iu et al .…”

“…In all, these data suggest that Sloth1 and Sloth2 localize to mitochondria and are important to support respiration and ATP production. Sloth1/2 regulate respiratory complex III assembly While our study was in preparation, two studies demonstrated that human SMIM4 and C12orf73 are inner mitochondrial membrane peptides important for complex III assembly and physically interact with complex III subunits (ZHANG et al 2020;DENNERLEIN et al 2021). If Sloth1 or Sloth2 have similar roles in Drosophila, this could explain why sloth1/2 mutant flies have reduced ATP production.…”