Accessory Subunits of the Matrix Arm of Mitochondrial Complex I with a Focus on Subunit NDUFS4 and Its Role in Complex I Function and Assembly

Abstract

NADH:ubiquinone-oxidoreductase (complex I) is the largest membrane protein complex of the respiratory chain. Complex I couples electron transfer to vectorial proton translocation across the inner mitochondrial membrane. The L shaped structure of complex I is divided into a membrane arm and a matrix arm. Fourteen central subunits are conserved throughout species, while some 30 accessory subunits are typically found in eukaryotes. Complex I dysfunction is associated with mutations in the nuclear and mitochondrial genome, resulting in a broad spectrum of neuromuscular and neurodegenerative diseases. Accessory subunit NDUFS4 in the matrix arm is a hot spot for mutations causing Leigh or Leigh-like syndrome. In this review, we focus on accessory subunits of the matrix arm and discuss recent reports on the function of accessory subunit NDUFS4 and its interplay with NDUFS6, NDUFA12, and assembly factor NDUFAF2 in complex I assembly.

The ins and outs of the flavin mononucleotide cofactor of respiratory complex I

“…61 Accumulation of the stalled P/Q assembly intermediate with the associated assembly factor NDUFAF2 seems to be a common finding in mitochondria from patients carrying mutations in the NDUFS4, 34,62-64 NDUFV1, 64,65 NDUFS1, 66 NDUFS6, 34,67 and NDUFA2 68 genes. Moreover, the same blue native migration pattern is observed in mitochondria from NDUFS1, NDUFA2, NDUFS6, NDUFA6 knockout cell lines 8,36 and in a Ndufs4 knockout mouse model. 69 These evidences suggest that a single defect in one of the N-module subunits is sufficient to impair the incorporation of the others all-at-once, while the rest of the complex is assembled normally.…”

“…Eventually, the incorporation of this hydrogenase-like building block into the Q/P intermediate 33,35 activates the NADH:ubiquinone oxidoreductase activity. The remainder of the N-module accessory subunits joins lastly: NDUFA12, NDUFS6, and NDUFS4 first 36 and then NDUFA6, NDUFA7, and NDUFV3 (Figure 2). Therefore, during complex I maturation, the N-module is incorporated only at the last stage, when most of the enzyme is already assembled into a Q/P subcomplex (Figure 2).…”

The Mysterious Multitude: Structural Perspective on the Accessory Subunits of Respiratory Complex I

“…NDUFS4, NDUFS6 and NDUFA12 are assembled at the N/Q interface ( Figures 3A–C ) during the final stages of CI assembly in mammals ( Figure 2A ) ( Guerrero-Castillo et al, 2017a ). Each of these three subunits are important for the activity of CI with many associated disease-causing mutations [recently reviewed by ( Kahlhöfer et al, 2021 )].…”

“…The dependence on NDUFS4 for the incorporation of NDUFA12 was not seen in Y. lipolytica ΔNDUFS4 as this structure showed clear incorporation of NDUFA12 ( Parey et al, 2019 ). However, although assembly goes to completion and NDUFA12 is incorporated into the complex, ΔNDUFS4 resulted in an increase of bound assembly factor NDUFAF2 ( Parey et al, 2019 ; Kahlhöfer et al, 2021 ) indicating that the deletion introduces a bottleneck in the final stages of assembly ( Kahlhöfer et al, 2021 ).…”

Impaired Mitochondrial Bioenergetics under Pathological Conditions

“…In mammalian mitochondria, Complex I is the largest respiratory complex, with 31 additional supernumerary subunits. Even if the accessory subunits are not necessary for enzyme activity, Zickermann's group and Vik's group discuss the mutations in these subunits which modify the Complex I assembly and lead to detrimental enzyme activity associated with many mitochondrial disease states [5,6]. A novel mutation of human mtDNA in the ATP6 gene has detrimental consequences on ATP synthase on yeast, with a pathogenicity resembling that which compromises human health [7].…”