Mimicking a SURF1 allele reveals uncoupling of cytochrome c oxidase assembly from translational regulation in yeast

Mitochondrial dysfunction in humans is often caused by defects in the assembly process of the respiratory chain, leading to severe multisystem disorders. Leigh Syndrome, a neurodegenerative disorder of infancy, characterized by necrotic lesions in the brain, can be caused by mutations of mitochondrial or nuclear DNA. Nuclear gene mutations leading to Leigh Syndrome most frequently affect SURF1. SURF1 is a highly conserved assembly factor of the cytochrome c oxidase (COX), the terminal enzyme complex of the respiratory chain. The molecular function of SURF1 is still not completely understood but has been extensively studied in its yeast homolog, Shy1. This protein is involved in the maturation of early assembly intermediate complexes of COX, the accumulation of which negatively regulate translation of the central subunit Cox1. We have analyzed missense mutations that affect highly conserved amino acids, identified in Leigh Syndrome patients. We show that G¹²⁴ mutations lead to SURF1 precursor proteins that are effectively imported into mitochondria but undergo rapid turnover. In contrast, an Y²⁷⁴D exchange has no effect on import or stability of SURF1 but leads to impaired function of the protein, which accumulates in a 200 kDa COX assembly intermediate complex. In yeast, Shy1^Y344D does not lead to an accumulation of COX assembly intermediates, that link to feedback regulation of Cox1 expression. Instead, Cox1 expression is unaltered and COX assembly is impaired at later steps.

These data suggest uncoupling of Cox1 expression from the assembly of the cytochrome c oxidase when Shy1^Y344 is mutated and provide evidence for the dual molecular role of Shy1.