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Translocation of precursor proteins into the mitochondrial matrix occurs through an environment accessible to aqueous
E S Sztul1, T W Chu, A W Strauss
1Yale University School of Medicine, Department of Human Genetics, New Haven 06510.
Abstract:
We have identified translocational intermediates generated during import of precursor proteins into the mitochondrial matrix and have characterized their association with mitochondrial membranes. Partially translocated forms of mitochondrial malate dehydrogenase (MDH) and ornithine transcarbamylase (OTC) were generated during import of the corresponding precursors (pMDH and pOTC) into mitochondria at 2 degrees C. Import at this temperature results in the formation of intermediate-sized MDH (iMDH) and OTC (iOTC) produced by the removal of a portion of the leader peptide, and in the production of mature-sized MDH. All of these forms contain NH2 termini located within the mitochondrial matrix, although the majority of their polypeptide chains remain extramitochondrial. All three are strongly associated with mitochondrial membranes, but can be extracted by protein denaturants such as urea. These translocational intermediates appear to be hydrophilic proteins, on the basis of their partitioning properties during extraction with the nonionic detergent Triton X-114. The data indicate that the translocation of polypeptide chains into mitochondria occurs in a microenvironment that is aqueous in nature and is mediated by integral membrane proteins.
Insights
Researchers identified protein intermediates during mitochondrial import, revealing they associate with membranes and have matrix-exposed N-termini. This suggests an aqueous translocation process mediated by membrane proteins.
Area of Science:
- Biochemistry
- Cell Biology
- Mitochondrial Biology
Background:
- Mitochondrial protein import is crucial for cellular function.
- Understanding the translocation process is key to deciphering mitochondrial biogenesis.
Purpose of the Study:
- To identify and characterize translocational intermediates during protein import into the mitochondrial matrix.
- To elucidate the association of these intermediates with mitochondrial membranes.
Main Methods:
- Protein import into isolated mitochondria at low temperature (2°C).
- Analysis of partially translocated proteins (iMDH, iOTC) and mature forms.
- Extraction of intermediates using protein denaturants (urea) and nonionic detergents (Triton X-114).
Main Results:
- Identified intermediate-sized mitochondrial malate dehydrogenase (iMDH) and ornithine transcarbamylase (iOTC) during import.
- These intermediates possess matrix-exposed N-termini but extramitochondrial polypeptide chains.
- Intermediates strongly associate with mitochondrial membranes and exhibit hydrophilic properties.
Conclusions:
- Mitochondrial protein translocation occurs via intermediates associated with membranes.
- The translocation process likely involves an aqueous microenvironment.
- Integral membrane proteins mediate the translocation of polypeptide chains into mitochondria.