A general mechanism for regulation of access to the translocon: Competition for a membrane attachment site on ribosomes

A general mechanism for regulation of access to the translocon: Competition for a membrane attachment site on ribosomes

^*Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10021; ^†Medizinische Biochemie, Universität des Saarlandes, D-66421 Homburg, Germany; ^‡Department of Cell Biology, New York University School of Medicine, New York, NY 10016; ^¶Cornell University Graduate School of Medical Sciences, New York, NY 10021; and ^‖Department of Pathology, New York Hospital/Cornell Medical Center, New York, NY 10021

Communicated by James E. Rothman, Memorial Sloan-Kettering Cancer Center, New York, NY (received for review July 21, 1998)

Abstract

For proteins to enter the secretory pathway, the membrane attachment site (M-site) on ribosomes must bind cotranslationally to the Sec61 complex present in the endoplasmic reticulum membrane. The signal recognition particle (SRP) and its receptor (SR) are required for targeting, and the nascent polypeptide associated complex (NAC) prevents inappropriate targeting of nonsecretory nascent chains. In the absence of NAC, any ribosome, regardless of the polypeptide being synthesized, binds to the endoplasmic reticulum membrane, and even nonsecretory proteins are translocated across the endoplasmic reticulum membrane. By occupying the M-site, NAC prevents all ribosome binding unless a signal peptide and SRP are present. The mechanism by which SRP overcomes the NAC block is unknown. We show that signal peptide-bound SRP occupies the M-site and therefore keeps it free of NAC. To expose the M-site and permit ribosome binding, SR can pull SRP away from the M-site without prior release of SRP from the signal peptide.

Footnotes

↵ § To whom reprint requests should be addressed at: Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021.
ABBREVIATIONS:

ER,

endoplasmic reticulum;

M-site,

membrane attachment site;

SRP,

signal recognition particle;

NAC,

nascent polypeptide associated complex;

SR,

SRP receptor;

RNC,

ribosome nascent chain complex;

pPL,

preprolactin;

eq,

equivalent;

ffLuc,

firefly luciferase