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Vol. 9, Issue 1, 103-115, January 1998
and
*Howard Hughes Medical Institute and Department of Cell Biology,
Harvard Medical School, Boston, Massachusetts 02115; and
Most secretory and membrane proteins are sorted by signal sequences
to the endoplasmic reticulum (ER) membrane early during their
synthesis. Targeting of the ribosome-nascent chain complex (RNC)
involves the binding of the signal sequence to the signal recognition
particle (SRP), followed by an interaction of ribosome-bound SRP with
the SRP receptor. However, ribosomes can also independently bind to the
ER translocation channel formed by the Sec61p complex. To explain the
specificity of membrane targeting, it has therefore been proposed that
nascent polypeptide-associated complex functions as a cytosolic
inhibitor of signal sequence- and SRP-independent ribosome binding to
the ER membrane. We report here that SRP-independent binding of RNCs to
the ER membrane can occur in the presence of all cytosolic factors,
including nascent polypeptide-associated complex. Nontranslating
ribosomes competitively inhibit SRP-independent membrane binding of
RNCs but have no effect when SRP is bound to the RNCs. The protective
effect of SRP against ribosome competition depends on a functional
signal sequence in the nascent chain and is also observed with
reconstituted proteoliposomes containing only the Sec61p complex and
the SRP receptor. We conclude that cytosolic factors do not prevent the
membrane binding of ribosomes. Instead, specific ribosome targeting to
the Sec61p complex is provided by the binding of SRP to RNCs, followed
by an interaction with the SRP receptor, which gives RNC-SRP complexes
a selective advantage in membrane targeting over nontranslating
ribosomes.
Max Delbrueck Center for Molecular Medicine, Robert
Roessle Strasse 10, 13122 Berlin-Buch, Germany
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