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Vol. 9, Issue 1, 117-130, January 1998
Department of Biochemistry and Molecular Biology, University of
Massachusetts Medical School, Worcester, Massachusetts 01655-0103
Proteins with RER-specific signal sequences are cotranslationally
translocated across the rough endoplasmic reticulum through a
proteinaceous channel composed of oligomers of the Sec61 complex. The
Sec61 complex also binds ribosomes with high affinity. The dual
function of the Sec61 complex necessitates a mechanism to prevent
signal sequence-independent binding of ribosomes to the translocation
channel. We have examined the hypothesis that the signal recognition
particle (SRP) and the nascent polypeptide-associated complex (NAC),
respectively, act as positive and negative regulatory factors to
mediate the signal sequence-specific attachment of the ribosome-nascent
chain complex (RNC) to the translocation channel. Here, SRP-independent
translocation of a nascent secretory polypeptide was shown to occur in
the presence of endogenous wheat germ or rabbit reticulocyte NAC.
Furthermore, SRP markedly enhanced RNC binding to the translocation
channel irrespective of the presence of NAC. Binding of RNCs, but not
SRP-RNCs, to the Sec61 complex is competitively inhibited by 80S
ribosomes. Thus, the SRP-dependent targeting pathway provides a
mechanism for delivery of RNCs to the translocation channel that is not
inhibited by the nonselective interaction between the ribosome and the
Sec61 complex.
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