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  • Title: Empty site forms of the SRP54 and SR alpha GTPases mediate targeting of ribosome-nascent chain complexes to the endoplasmic reticulum.
    Author: Rapiejko PJ, Gilmore R.
    Journal: Cell; 1997 May 30; 89(5):703-13. PubMed ID: 9182758.
    Abstract:
    The SRP54 and SR alpha subunits of the signal recognition particle (SRP) and the SRP receptor (SR) undergo a tightly coupled GTPase cycle that mediates the signal sequence-dependent attachment of ribosomes to the Sec61 complex. Here, we show that SRP54 and SR alpha are in the empty site conformation prior to contact between the SRP-ribosome complex and the membrane-bound SR. Cooperative binding of GTP to SRP54 and SR alpha stabilizes the SRP-SR complex and initiates signal sequence transfer from SRP54 to Sec61 alpha. The GTP-bound conformations of SR alpha and SRP54 perform distinct roles, with SR alpha performing a predominant role in complex stabilization. Hydrolysis by both SRP54 and SR alpha is a prerequisite for dissociation of the SRP-SR complex.
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