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Correspondence to: Bernhard Dobberstein, Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), Postfach 106249, 69052 Heidelberg, Germany. Tel:(6221) 546825 Fax:(6221) 545892 E-mail: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
- and ß-subunit of the SRP receptor (SR). Here, we report on the GTPase cycle of the ß-subunits of the SR (SRß). We found that SRß binds GTP with high affinity and interacts with ribosomes in the GTP-bound state. Subsequently, the ribosome increases the GTPase activity of SRß and thus functions as a GTPase activating protein for SRß. Furthermore, the interaction between SRß and the ribosome leads to a reduction in the affinity of SRß for guanine nucleotides. We propose that SRß regulates the interaction of SR with the ribosome and thereby allows SR
to scan membrane-bound ribosomes for the presence of SRP. Interaction between SRP and SR
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
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