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The mitochondrial processing peptidase: function and specificity
1Laboratoire d'Ingéniérie des Systèmes Macromoléculaires, CNRS, Marseille, France.
Abstract:
Targeting signals of mitochondrial precursors are cleaved in the matrix during or after import by the mitochondrial processing peptidase (MPP). This enzyme consists of two nonidentical alpha- and beta-subunits each of molecular weight of about 50 kDa. In mammals and fungi, MPP is soluble in the matrix, whereas in plants the enzyme is part of the cytochrome bc1 complex. MPP is a metalloendopeptidase which has been classified as a member of the pitrilysin family on the basis of the HXXEHX76E zinc-binding motif present in beta-MPP. Both subunits of MPP are required for processing activity. The alpha-subunit of MPP, which probably recognizes a three-dimensional motif adopted by the presequence, presents the presequence to beta-MPP, which carries the catalytic active site. MPP acts as an endoprotease on chemically synthesized peptides corresponding to mitochondrial presequences. Matrix-targeting signals and MPP cleavage signals seem to be distinct, although the two signals may overlap within a given presequence. The structural element helix-turn-helix, that cleavable presequences adopt in a membrane mimetic environment, may be required for processing but is not sufficient for proteolysis. Binding of the presequence by alpha-MPP tolerates a high degree of mutations of the presequence. alpha-MPP may present a degenerated cleavage site motif to beta-MPP in an accessible conformation for processing. The conformation of mitochondrial presequences bound to MPP remains largely unknown.
Insights
Mitochondrial processing peptidase (MPP) cleaves targeting signals on mitochondrial precursors. The alpha-subunit recognizes presequences, presenting them to the catalytic beta-subunit for processing.
Area of Science:
- Mitochondrial biology
- Enzymology
- Protein processing
Background:
- Mitochondrial precursor proteins require targeting signals for import.
- Mitochondrial processing peptidase (MPP) removes these signals in the mitochondrial matrix.
- MPP is a heterodimeric metalloendopeptidase crucial for mitochondrial function.
Purpose of the Study:
- To elucidate the distinct roles of MPP's alpha and beta subunits in processing mitochondrial presequences.
- To investigate the interaction between MPP and mitochondrial targeting/cleavage signals.
- To understand the structural requirements for MPP-mediated presequence cleavage.
Main Methods:
- Enzymatic assays using chemically synthesized peptides corresponding to mitochondrial presequences.
- Analysis of MPP's conserved zinc-binding motif (HXXEHX76E) in the beta-subunit.
- Investigation of presequence structure (e.g., helix-turn-helix) in membrane-mimetic environments.
Main Results:
- Both alpha and beta subunits are essential for MPP's catalytic activity.
- The alpha-subunit likely recognizes a three-dimensional motif of the presequence, while the beta-subunit contains the active site.
- Cleavage signals are distinct from targeting signals, though they may overlap.
- A helix-turn-helix motif is necessary but not sufficient for cleavage.
- Alpha-MPP binding tolerates presequence mutations, potentially presenting degenerate cleavage sites to beta-MPP.
Conclusions:
- MPP functions as an endoprotease with specialized roles for its alpha and beta subunits.
- The precise conformation of presequences bound to MPP remains largely uncharacterized.
- Understanding MPP function is key to comprehending mitochondrial protein import and regulation.