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N-terminal hydrophobic sorting signals of preproteins confer mitochondrial hsp70 independence for import into
A Gruhler1, I Arnold, T Seytter
1Institut für Physiologische Chemie, Universität München, Goethestrasse 33, 80336 München, Federal Republic of Germany.
Abstract:
The requirement of mitochondrial hsp70 (mt-hsp70) for the import of a series of preproteins containing hydrophobic sorting signals into isolated yeast mitochondria was investigated. Here we demonstrate that the presence of such a sorting signal in proximity to the N-terminal matrix-targeting sequence of a preprotein can secure a translocating polypeptide chain in the import channel in a manner that does not require mt-hsp70 activity. Trapping the translocating chain in this fashion leads to efficient processing by the mitochondrial processing peptidase and to complete translocation across the outer mitochondrial membrane into the intermembrane space. These mt-hsp70-independent effects appear to be exerted at the level of the inner membrane through an interaction of the hydrophobic core of the sorting signal with component(s) of the translocase of the inner membrane. Hydrophobic sorting signals of inner membrane proteins inserted into the membrane from the matrix, as well as those of intermembrane space proteins, are capable of causing this mt-hsp70-independent stabilization, demonstrating that this phenomenon is not unique to those preproteins normally sorted to the intermembrane space.
Insights
Mitochondrial heat shock protein 70 (mtHsp70) is not always required for protein import into yeast mitochondria. Hydrophobic sorting signals can independently facilitate protein translocation across mitochondrial membranes.
Area of Science:
- Mitochondrial biology
- Protein import
- Cellular transport
Background:
- Mitochondrial heat shock protein 70 (mtHsp70) is crucial for importing proteins into mitochondria.
- The precise mechanisms by which proteins with hydrophobic sorting signals are imported are not fully understood.
Purpose of the Study:
- To investigate the role of mtHsp70 in the import of yeast mitochondrial preproteins with hydrophobic sorting signals.
- To determine if hydrophobic sorting signals can mediate mtHsp70-independent protein import.
Main Methods:
- Utilized isolated yeast mitochondria and preproteins with specific N-terminal targeting sequences and hydrophobic sorting signals.
- Analyzed the translocation and processing of these preproteins in the presence and absence of mtHsp70 activity.
Main Results:
- Hydrophobic sorting signals, when positioned near the N-terminal targeting sequence, can trap preproteins in the import channel, independent of mtHsp70.
- This trapping facilitates efficient processing and complete translocation across the outer mitochondrial membrane into the intermembrane space.
- The effect involves interaction with components of the inner mitochondrial membrane translocase.
Conclusions:
- mtHsp70-independent protein import into mitochondria is possible.
- Hydrophobic sorting signals play a significant role in mediating protein translocation, particularly at the inner mitochondrial membrane.
- This mechanism applies to both intermembrane space and inner membrane proteins, highlighting a conserved import pathway.