Differential cysteine depletion in respiratory chain complexes enables the distinction of longevity from aerobicity

Mitochondrially encoded proteins in long-lived animals exhibit a characteristic anomaly on the amino acid usage level: they abstain from the use of cysteine in a lifespan-dependent fashion. Here, we have further investigated this phenomenon by analyzing respiratory chain complex subunits individually. We find that complex I cysteine depletion is the almost exclusive carrier of the cysteine-lifespan correlation, whereas complex IV cysteine depletion is uniform in all aerobic animals, unrelated to longevity, but even more pronounced than complex I cysteine depletion in the longest-lived species. In nuclear encoded subunits of the respiratory chain, we find lifespan-independent cysteine depletion ranging from moderate in complex I to severe in complex V. However, a separate analysis of the transmembrane domains of these proteins unmasks an intramembrane pattern of cysteine usage that significantly correlates with longevity. Hence, cysteine usage in the respiratory chain seems to be governed by two independent molecular mechanisms acting on the protein level: a global trend of cysteine avoidance in all proteins, and a specific effect on transmembrane domain cysteines that reflects longevity. To account for these characteristic patterns, a thiyl radical-based molecular mechanism of intramembrane hydrophobic cross-linking is discussed.