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Published online 4 November 2002. doi:10.1083/jcb.200204144
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© The Rockefeller University Press, 0021-9525/2002/11/403 $5.00
The Journal of Cell Biology, Volume 159, Number 3, 403-410

Report

Accumulation of endoplasmic membranes and novel membrane-bound ribosome–signal recognition particle receptor complexes in Escherichia coli



Anat A. Herskovits1, Eyal Shimoni2, Abraham Minsky3 and Eitan Bibi1

1 Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel
2 Electron Microscopy Unit, Weizmann Institute of Science, Rehovot 76100, Israel
3 Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel

Address correspondence to Eitan Bibi, Dept. of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel. Tel./Fax: 972-8-934-3464. E-mail: bcbibi{at}wicc.weizmann.ac.il

In Escherichia coli, ribosomes must interact with translocons on the membrane for the proper integration of newly synthesized membrane proteins, cotranslationally. Previous in vivo studies indicated that unlike the E. coli signal recognition particle (SRP), the SRP receptor FtsY is required for membrane targeting of ribosomes. Accordingly, a putative SRP-independent, FtsY-mediated ribosomal targeting pathway has been suggested (Herskovits, A.A., E.S. Bochkareva, and E. Bibi. 2000. Mol. Microbiol. 38:927–939). However, the nature of the early contact of ribosomes with the membrane, and the involvement of FtsY in this interaction are unknown. Here we show that in cells depleted of the SRP protein, Ffh or the translocon component SecE, the ribosomal targeting pathway is blocked downstream and unprecedented, membrane-bound FtsY–ribosomal complexes are captured. Concurrently, under these conditions, novel, ribosome-loaded intracellular membrane structures are formed. We propose that in the absence of a functional SRP or translocon, ribosomes remain jammed at their primary membrane docking site, whereas FtsY-dependent ribosomal targeting to the membrane continues. The accumulation of FtsY-ribosome complexes induces the formation of intracellular membranes needed for their quantitative accommodation. Our results with E. coli, in conjunction with recent observations made with the yeast Saccharomyces cerevisiae, raise the possibility that the SRP receptor–mediated formation of intracellular membrane networks is governed by evolutionarily conserved principles.

Key Words: E. coli; signal recognition particle; ribosome; protein targeting; membrane proteins


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