Tissue-specific cytochrome c oxidase assembly defects due to mutations in SCO2 and SURF1

Abstract

The biogenesis of eukaryotic COX (cytochrome c oxidase) requires several accessory proteins in addition to structural subunits and prosthetic groups. We have analysed the assembly state of COX and SCO2 protein levels in various tissues of six patients with mutations in SCO2 and SURF1. SCO2 is a copper-binding protein presumably involved in formation of the Cu(A) centre of the COX2 subunit. The function of SURF1 is unknown. Immunoblot analysis of native gels demonstrated that COX holoenzyme is reduced to 10-20% in skeletal muscle and brain of SCO2 and SURF1 patients and to 10-30% in heart of SCO2 patients, whereas liver of SCO2 patients' contained normal holoenzyme levels. The steady-state levels of mutant SCO2 protein ranged from 0 to 20% in different SCO2 patient tissues. In addition, eight distinct COX subcomplexes and unassembled subunits were found, some of them identical with known assembly intermediates of the human enzyme. Heart, brain and skeletal muscle of SCO2 patients contained accumulated levels of the COX1.COX4.COX5A subcomplex, three COX1-containing subcomplexes, a COX4.COX5A subcomplex and two subcomplexes composed of only COX4 or COX5A. The accumulation of COX1.COX4.COX5A subcomplex, along with the virtual absence of free COX2, suggests that the lack of the Cu(A) centre may result in decreased stability of COX2. The appearance of COX4.COX5A subcomplex indicates that association of these nucleus-encoded subunits probably precedes their addition to COX1 during the assembly process. Finally, the consequences of SCO2 and SURF1 mutations suggest the existence of tissue-specific functional differences of these proteins that may serve different tissue-specific requirements for the regulation of COX biogenesis.

Formation of the Redox Cofactor Centers during Cox1 Maturation in Yeast Cytochrome Oxidase

“…Studies with fibroblasts from patients with mutant Cox10 or Cox15 reveal limited accumulation of the free Cox1 subunit (1,2). In contrast, CcO-deficient patients with mutations in SURF1 revealed a Cox1 assembly intermediate with two nuclear CcO subunits, CoxIV and Va (equivalent to yeast subunits Cox5a and Cox6) (39,43,47). One interpretation of these results is that heme a insertion may be necessary for formation or stabilization of the S2 intermediate.…”

hCOA3 Stabilizes Cytochrome c Oxidase 1 (COX1) and Promotes Cytochrome c Oxidase Assembly in Human Mitochondria

“…For example, SCO1 and SCO2 mutant fibroblasts, both of which are required for COX2 maturation, accumulate the S2 intermediate (29,36). In the hCOA3-silenced cells, the accumulation of the S2 intermediate indicates hCOA3 is a COX assembly factor involved in the first steps of the assembly process, probably facilitating the incorporation of COX subunits to COX1-containing intermediates.…”

Analysis of Leigh Syndrome Mutations in the Yeast SURF1 Homolog Reveals a New Member of the Cytochrome Oxidase Assembly Factor Family

“…One intermediate, designated S2, which accumulates in SURF1-deficient patient fibroblasts, consists of Cox1 in association with two nuclear CcO subunits, CoxIV and Va (38,45,47). A similar stalled assembly intermediate accumulates in CcO-deficient patients with mutations in two other assembly factors, SCO1 and SCO2.…”