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The Intrinsic Ability of Ribosomes to Bind to Endoplasmic Reticulum Membranes is Regulated by Signal Recognition Particle and Nascent- Polypeptide-Associated Complex
Brett Lauring, Gert Kreibich and Martin Wiedmann
Proceedings of the National Academy of Sciences of the United States of America
Vol. 92, No. 21 (Oct. 10, 1995), pp. 9435-9439
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/2368490
Page Count: 5
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Signal peptides direct the cotranslational targeting of nascent polypeptides to the endoplasmic reticulum (ER). It is currently believed that the signal recognition particle (SRP) mediates this targeting by first binding to signal peptides and then by directing the ribosome/nascent chain/SRP complex to the SRP receptor at the ER. We show that ribosomes can mediate targeting by directly binding to translocation sites. When purified away from cytosolic factors, including SRP and nascent-polypeptide-associated complex (NAC), in vitro assembled translation intermediates representing ribosome/nascent-chain complexes efficiently bound to microsomal membranes, and their nascent polypeptides could subsequently be efficiently translocated. Because removal of cytosolic factors from the ribosome/nascent-chain complexes also resulted in mistargeting of signalless nascent polypeptides, we previously investigated whether readdition of cytosolic factors, such as NAC and SRP, could restore fidelity to targeting. Without SRP, NAC prevented all nascent-chain containing ribosomes from binding to the ER membrane. Furthermore, SRP prevented NAC from blocking ribosome-membrane association only when the nascent polypeptide contained a signal. Thus, NAC is a global ribosome-binding prevention factor regulated in activity by signal-peptidedirected SRP binding. A model presents ribosomes as the targeting vectors for delivering nascent polypeptides to translocation sites. In conjunction with signal peptides, SRP and NAC contribute to this specificity of ribosomal function by regulating exposure of a ribosomal membrane attachment site that binds to receptors in the ER membrane.
Proceedings of the National Academy of Sciences of the United States of America © 1995 National Academy of Sciences