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Discrimination between SRP- and SecA/SecB-dependent substrates involves selective recognition of nascent chains by
1Institut für Biochemie und Molekularbiologie und, Universität Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.
The EMBO Journal
|January 5, 2000
Summary
Escherichia coli uses signal recognition particle (SRP) for co-translational targeting of polytopic membrane proteins to the SecY translocon. Secretory proteins, however, are directed to the SecA/SecB pathway by trigger factor.
Area of Science:
- Microbiology
- Molecular Biology
- Protein Targeting
Background:
- Escherichia coli utilizes SecA, SecB, and signal recognition particle (SRP) for protein export.
- The SecY translocon is a key component in protein translocation across membranes.
Purpose of the Study:
- To elucidate the distinct targeting pathways for polytopic membrane proteins and secretory proteins in E. coli.
- To investigate the roles of SRP, trigger factor, SecA, and SecB in these pathways.
Main Methods:
- In vitro chemical and site-specific cross-linking experiments.
- Analysis of interactions between nascent polypeptides, targeting factors, and the SecY translocon.
Main Results:
- SRP recognizes the signal anchor sequence of polytopic membrane protein MtlA, enabling co-translational targeting to SecY and phospholipids.
- Trigger factor association with nascent pOmpA prevents SRP interaction and also inhibits SecA binding.
- SecA does not interact with MtlA, and trigger factor shows minimal binding to ribosome-bound MtlA.
Conclusions:
- Polytopic membrane proteins are selected by SRP for co-translational targeting to membranes.
- Secretory proteins are channeled into the SecA/SecB post-translational pathway via preferential recognition by trigger factor.