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© The Rockefeller University Press, 0021-9525/1999//723 $5.00
The Journal of Cell Biology, Volume 146, Number 4, , 1999 723-730

Original Article

The Ribosome Regulates the Gtpase of the β-Subunit of the Signal Recognition Particle Receptor



Gerald Bachera, Martin Poola, and Bernhard Dobbersteina

a Zentrum für Molekulare Biologie der Universität Heidelberg, D-69052 Heidelberg, Germany
Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), Postfach 106249, 69052 Heidelberg, Germany.(6221) 545892(6221) 546825

dobberstein{at}zmbh.uni-heidelberg.de

Protein targeting to the membrane of the ER is regulated by three GTPases, the 54-kD subunit of the signal recognition particle (SRP) and the {alpha}- and β-subunit of the SRP receptor (SR). Here, we report on the GTPase cycle of the β-subunits of the SR (SRβ). We found that SRβ binds GTP with high affinity and interacts with ribosomes in the GTP-bound state. Subsequently, the ribosome increases the GTPase activity of SRβ and thus functions as a GTPase activating protein for SRβ. Furthermore, the interaction between SRβ and the ribosome leads to a reduction in the affinity of SRβ for guanine nucleotides. We propose that SRβ regulates the interaction of SR with the ribosome and thereby allows SR{alpha} to scan membrane-bound ribosomes for the presence of SRP. Interaction between SRP and SR{alpha} then leads to release of the signal sequence from SRP and insertion into the translocon. GTP hydrolysis then results in dissociation of SR from the ribosome, and SRP from the SR.

Key Words: signal recognition particle receptor • GTPase • ribosome • translocation • endoplasmic reticulum

© 1999 The Rockefeller University Press

1.used in this paper: GAP, GTPase activating protein; GEFs, guanine nucleotide exchange factors; PPL, preprolactin; RNC, ribosome–nascent chain complex; SR, signal recognition particle receptor; SR{alpha}, {alpha}-subunit of the SR; SRβ, β-subunit of the SR; SRP, signal recognition particle

Gerald Bacher's present address is ASTA Medica AG, Weismuellerstr. 45, 60314 Frankfurt am Main, Germany.


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