Protein targeting to the endoplasmic reticulum (ER) membrane is regulated by three GTPases, the 54 kDa subunit of the signal recognition particle (SRP) and the alpha- and beta-subunits of the SRP receptor (SR). Using a soluble form of SR and an XTP-binding mutant of SRbeta, we show that SRbeta is essential for protein translocation across the ER membrane. SRbeta can be cross-linked to a 21 kDa ribosomal protein in its empty and GDP-bound state, but not when GTP is bound. GTP binding to SRbeta...Expand abstract
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SRbeta coordinates signal sequence release from SRP with ribosome binding to the translocon.
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