Human cells maintain mitochondrial protein stability through a precise trimming mechanism driven by an enzyme called ICP55, according to a study published in Nature Structural & Molecular Biology. Removing a single amino acid at the protein N-terminus prevents large complexes like respiratory chain units and HSP60 from unraveling into smaller parts.
Mitochondria rely on a delicate choreography of protein import and assembly to sustain cellular energy production. Most mitochondrial proteins originate outside the organelle and carry an initial address sequence at their N-terminus. Once these proteins enter the mitochondrion, specialized enzymes remove this address, setting the stage for structural assembly.
Mapping ICP55 Protein Targets in Human Mitochondria
Researchers led by Dr. Nora Vögtle of Heidelberg University and Dr. Pitter Huesgen of the University of Freiburg examined this maturation pathway by mapping the mature N-termini of mitochondrial proteins using proteomic methods. According to the study, scientists identified 446 mature mitochondrial N-termini and pinpointed 107 specific proteins targeted for processing by ICP55. After another enzyme strips away the initial address sequence, ICP55 cleaves off exactly one additional amino acid. To uncover what happens when this second step fails, Vögtle’s group generated human cells lacking the enzyme entirely.
Complexome Profiling Reveals Unraveling Protein Complexes
Without ICP55, large protein structures lose their integrity. Using complexome profiling, the researchers tracked how proteins organized within mitochondria in the absence of the enzyme. Instead of forming fully assembled units, proteins appeared more frequently as individual subunits or in smaller, unstable complexes. The disrupted structures included respiratory chain complexes, the HSP60 complex responsible for protein folding, mitochondrial ribosomes, and various metabolic protein complexes.
We were surprised to find that removing a single amino acid has such a fundamental impact on the stability of mitochondrial protein complexes. This means that the precise trimming of the N-terminus is crucial for proteins to subsequently assemble into stable complexes.
Dr. Nora Vögtle, Center for Molecular Biology at Heidelberg University
To confirm that this single cleavage dictates stability, the researchers tested HSP60 directly. Complexes that had already formed broke apart in a test tube when the precise trimming was absent, driving up the count of free HSP60 protein subunits.
Implications for Mitochondrial Disease Biology
Mitochondrial dysfunction already underpins more than 50 different disorders, ranging from neonatal fatalities to adult-onset neurodegeneration, alongside contributions to cancer and type II diabetes. In particular, complex I deficiency stands as the most common cause of rare respiratory chain diseases and has been implicated in Parkinson’s disease. With half of complex I deficiency patients lacking mutations in known subunits, researchers continue searching for unmapped factors crucial for complex assembly, stability, or maturation.
The newly identified mechanism provides a molecular starting point for understanding how microscopic errors in protein maturation can impair organelle function. The exact sequence of processing by ICP55 has been shown to be a fundamental prerequisite for the formation of stable functional units,
Huesgen noted, pointing toward future avenues for investigating mitochondrial diseases.