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Published online 11 August 2003. doi:10.1083/jcb.200303143
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© The Rockefeller University Press, 0021-9525/2003/8/575 $5.00
The Journal of Cell Biology, Volume 162, Number 4, 575-585

Article

Dual recognition of the ribosome and the signal recognition particle by the SRP receptor during protein targeting to the endoplasmic reticulum



Elisabet C. Mandon, Ying Jiang and Reid Gilmore

Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605

Address correspondence to Reid Gilmore, Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, 364 Plantation St., Worcester, MA 01605-2324. Tel.: (508) 856-5894. Fax: (508) 856-6464. email: reid.gilmore{at}umassmed.edu

We have analyzed the interactions between the signal recognition particle (SRP), the SRP receptor (SR), and the ribosome using GTPase assays, biosensor experiments, and ribosome binding assays. Possible mechanisms that could contribute to an enhanced affinity between the SR and the SRP–ribosome nascent chain complex to promote protein translocation under physiological ionic strength conditions have been explored. Ribosomes or 60S large ribosomal subunits activate the GTPase cycle of SRP54 and SR{alpha} by providing a platform for assembly of the SRP–SR complex. Biosensor experiments revealed high-affinity, saturable binding of ribosomes or large ribosomal subunits to the SR. Remarkably, the SR has a 100-fold higher affinity for the ribosome than for SRP. Proteoliposomes that contain the SR bind nontranslating ribosomes with an affinity comparable to that shown by the Sec61 complex. An NH2-terminal 319-residue segment of SR{alpha} is necessary and sufficient for binding of SR to the ribosome. We propose that the ribosome–SR interaction accelerates targeting of the ribosome nascent chain complex to the RER, while the SRP–SR interaction is crucial for maintaining the fidelity of the targeting reaction.

Key Words: protein translocation; signal recognition particle; ribosome; endoplasmic reticulum; biosensor

Abbreviations used in this paper: GAP, GTPase-activating protein; GEF, guanine nucleotide exchange factor; pPL, preprolactin; RM, rough microsome; RNC, ribosome nascent chain complex; SR, SRP receptor; SRP, signal recognition particle.


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