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J. Biol. Chem., Vol. 281, Issue 14, 9038-9048, April 7, 2006

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An Unusual Signal Peptide Extension Inhibits the Binding of Bacterial Presecretory Proteins to the Signal Recognition Particle, Trigger Factor, and the SecYEG Complex^*

From the Genetics and Biochemistry Branch, NIDDK, National Institutes of Health, Bethesda, Maryland 20892-0538

Considerable evidence indicates that the Escherichia coli signal recognition particle (SRP) selectively targets proteins that contain highly hydrophobic signal peptides to the SecYEG complex cotranslationally. Presecretory proteins that contain only moderately hydrophobic signal peptides typically interact with trigger factor (TF) and are targeted post-translationally. Here we describe a striking exception to this rule that has emerged from the analysis of an unusual 55-amino acid signal peptide associated with the E. coli autotransporter EspP. The EspP signal peptide consists of a C-terminal domain that resembles a classical signal peptide plus an N-terminal extension that is conserved in other autotransporter signal peptides. Although a previous study showed that proteins containing the C-terminal domain of the EspP signal peptide are targeted cotranslationally by SRP, we found that proteins containing the full-length signal peptide were targeted post-translationally via a novel TF-independent mechanism. Mutation of an invariant asparagine residue in the N-terminal extension, however, restored cotranslational targeting. Remarkably, proteins containing extremely hydrophobic derivatives of the EspP signal peptide were also targeted post-translationally. These and other results suggest that the N-terminal extension alters the accessibility of the signal peptide to SRP and TF and promotes post-translational export by reducing the efficiency of the interaction between the signal peptide and the SecYEG complex. Based on data, we propose that the N-terminal extension mediates an interaction with an unidentified cytoplasmic factor or induces the formation of an unusual signal peptide conformation prior to the onset of protein translocation.

Received for publication, August 8, 2005 , and in revised form, December 2, 2005.

^* This research was supported by the Intramural Research Program of NIDDK, National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Present address: Dept. of Medical Microbiology and Immunology, University of Wisconsin School of Medicine, Madison, WI 53706.

² To whom correspondence should be addressed: National Institutes of Health, Bldg. 5, Rm. 201, Bethesda, MD 20892-0538. Tel.: 301-402-4770; Fax: 301-496-9878; E-mail: harris_bernstein{at}nih.gov.

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