A small Tim homohexamer in the relict mitochondrion of Cryptosporidium

Area of Science:

• Parasitology
• Cell Biology
• Evolutionary Biology

Background:

• Apicomplexan parasites like Cryptosporidium parvum have relict organelles called mitosomes, derived from mitochondria.
• Mitosomes possess a reduced metabolic capacity but retain a protein import system, though parts have been lost during evolution.
• The small Tim protein family, typically found in pairs, is crucial for protein import into mitochondria and related organelles.

Purpose of the Study:

• To investigate the structure and evolutionary implications of the single small Tim protein (CpTimS) found in Cryptosporidium parvum.
• To challenge the established model of small Tim proteins always existing as heterodimers.
• To understand the formation of small Tim hexamers in eukaryotic protein import pathways.

Main Methods:

• Bioinformatic analysis of Cryptosporidium genomes to identify small Tim proteins.
• Biochemical characterization of the atypical CpTimS protein.
• Structural studies to determine the oligomeric state of CpTimS.

Main Results:

• Cryptosporidium parvum encodes only one small Tim protein, CpTimS, contradicting the usual presence of paired small Tims.
• CpTimS forms a relatively unstable homohexamer, not the expected heterodimer.
• This finding provides insights into the early evolution of the small Tim protein family.

Conclusions:

• The atypical CpTimS structure challenges the conserved requirement for heterodimeric small Tim proteins in eukaryotes.
• The study sheds light on the evolutionary plasticity of the protein import machinery in reduced organelles.
• Understanding CpTimS hexamer formation offers new perspectives on small Tim protein assembly and function in contemporary eukaryotes.