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Published December 1, 1983 | Published
Journal Article Open

Mechanisms of integration of de novo-synthesized polypeptides into membranes: Signal-recognition particle is required for integration into microsomal membranes of calcium ATPase and of lens MP26 but not of cytochrome b_5


We have investigated the in vitro integration into dog pancreas microsomal membranes of three integral membrane proteins that were synthesized de novo in a wheat germ cell-free translation system: calcium ATPase of rabbit sarcoplasmic reticulum, MP26 of bovine lens fiber plasma membrane, and rat liver cytochrome b_5. Biosynthetically these proteins show a common feature in that they are synthesized without a transient NH_2-terminal signal sequence. Two of these proteins, ATPase and MP26, were shown to require the recently discovered signal-recognition particle (SRP) [Walter, P. & Blobel, G. (1982) Nature (London) 299, 691-698] for integration. By this criterion, therefore, they each contain at least one uncleaved signal sequence. Surprisingly, however, the uncleaved signal sequence(s) of these two proteins did not induce the characteristic SRP-mediated translation arrest that was previously shown for a cleaved signal sequence. Unlike ATPase and MP26, cytochrome b_5 did not require SRP for integration into microsomal membrane. Thus, the distinction between an "insertion" sequence (specifying unassisted and opportunistic integration into any exposed membrane) and a "signal" sequence (directing integration into a specific membrane by a receptor-mediated mechanism) is a valid one. By assaying for SRP dependence, the two mechanisms of integration can now be experimentally distinguished.

Additional Information

© 1983 by the National Academy of Sciences. Contributed by Günter Blobel, August 31, 1983. We thank Peter Walter for helpful discussions and Ms. Gisele Nimic for typing the manuscript. This work was supported by National Institutes of Health Grant GM 27155. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S. C. §1734 solely to indicate this fact.

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