Sam50 Functions in Mitochondrial Intermembrane Space Bridging and Biogenesis of Respiratory Complexes

Abstract

f Mitochondria possess an outer membrane (OMM) and an inner membrane (IMM), which folds into invaginations called cristae. Lipid composition, membrane potential, and proteins in the IMM influence organization of cristae. Here we show an essential role of the OMM protein Sam50 in the maintenance of the structure of cristae. Sam50 is a part of the sorting and assembly machinery (SAM) necessary for the assembly of ␤-barrel proteins in the OMM. We provide evidence that the SAM components exist in a large protein complex together with the IMM proteins mitofilin and CHCHD3, which we term the mitochondrial intermembrane space bridging (MIB) complex. Interactions between OMM and IMM components of the MIB complex are crucial for the preservation of cristae. After destabilization of the MIB complex, we observed deficiency in the assembly of respiratory chain complexes. Long-term depletion of Sam50 influences the amounts of proteins from all large respiratory complexes that contain mitochondrially encoded subunits, pointing to a connection between the structural integrity of cristae, assembly of respiratory complexes, and/or the maintenance of mitochondrial DNA (mtDNA).

Mitofilin complexes: conserved organizers of mitochondrial membrane architecture

“…In human cells, mitofilin was found to interact with the sorting and assembly machinery of the outer membrane (SAM complex) that has a central role in the biogenesis of outer membrane proteins ( Figure 2B) (Xie et al , 2007 ;Darshi et al , 2011 ;Alkhaja et al , 2012 ;Ott et al , 2012 ). In yeast, interactions of mitofilincontaining protein complexes of the inner membrane with the SAM complex, the TOM complex, porin channels and Ugo1 have been reported (Figure 2A) (Harner et al , 2011 ;Hoppins et al , 2011 ;von der Malsburg et al , 2011 ;K ö rner et al, 2012 ;Zerbes et al , 2012 ).…”

“…The possibility of such an overlapping localization has been repeatedly addressed, although never comprehensively clarified (van der Klei et al , 1994 ;Perkins et al , 1997 ;Reichert and Neupert , 2002 ;Harner et al , 2011 ;K ö rner et al, 2012 ). Some studies have suggested that the integrity of crista junctions may depend on their anchoring to the outer mitochondrial membrane via mitofilin-containing contact sites (Harner et al , 2011 ;K ö rner et al, 2012 ;Ott et al , 2012 ). Along the same lines, downregulation of the SAM complex, one of the outer membrane interaction partners of mitofilin, was found to result in alterations of mitochondrial ultrastructure in human cultured cells and yeast ( K ö rner et al, 2012 ;Ott et al , 2012 ).…”

“…Some studies have suggested that the integrity of crista junctions may depend on their anchoring to the outer mitochondrial membrane via mitofilin-containing contact sites (Harner et al , 2011 ;K ö rner et al, 2012 ;Ott et al , 2012 ). Along the same lines, downregulation of the SAM complex, one of the outer membrane interaction partners of mitofilin, was found to result in alterations of mitochondrial ultrastructure in human cultured cells and yeast ( K ö rner et al, 2012 ;Ott et al , 2012 ). A detailed structure-function analysis of the yeast mitofilin protein led to the isolation of a truncated variant that interacts efficiently with both the SAM and TOM complexes in the outer membrane, whereas crista junctions are virtually lost (Zerbes et al , 2012 ).…”

“…Instead, it has a predicted N-myristoylation site and is peripherally associated to the intermembrane space side of the inner membrane (Harner et al , 2011 ;Hoppins et al , 2011 ;von der Malsburg et al , 2011 ). Human mitofilin was shown to interact with two different putatively myristoylated peripheral inner membrane proteins located in the intermembrane space, the twin CX 9 C proteins CHCHD3 (MINOS3) and CHCHD6 (CHCM1) ( Figure 2B) (Xie et al , 2007 ;Darshi et al , 2011 ;Alkhaja et al , 2012 ;An et al , 2012 ;Ott et al , 2012 ). Twin CX 9 C proteins are characterized by a cysteine-rich motif that forms two disulfide bonds required for stabilization of their eponymous coiled coil-helix-coiled coil-helix (CHCH) domain (Cavallaro , 2010 ).…”

“…Both moma-1 and immt-1 mutants additionally exhibit altered mitochondrial ultrastructures with dilated cristae and occasional concentric inner membrane whorls. MOMA-1 has similarity to the human mitochondrial protein APOOL (Apo-O-like/CXorf33) and the related Apolipoprotein O/My025 (Lamant et al , 2006 ;Head et al , 2011 ;Ott et al , 2012 ). Yeast cells express two proteins related to MOMA-1, Aim37 and Mio27, both of which are subunits of the MINOS complex (Figure 2A) (Head et al , 2011 ;van der Laan et al , 2012 ).…”