Characterization of the beta-subunit of the mammalian SRP receptor and its role in assembly of the SRP receptor

Abstract

<p>The eukaryotic signal recognition particle (SRP) receptor is a heterodimeric complex present on the endoplasmic reticulum (ER) membrane, and is required for the targeting and translocation of nascent polypeptide chains across the ER. Both SRP receptor subunits, SRα. and SRβ, are GTP-binding proteins. The role of the SRα subunit in the nascent chain targeting reaction is wen understood, but the function of the SRβ subunit has not been determined. This thesis demonstrates that a complete and functional GTP-binding domain of SRβ is necessary to bind SRα, and that the integrity of the SRα-SRβ dimer is regulated by the SRβ GTPase. Further analysis of SRI3 revealed functional characteristics that are not shared with other GTPases. Most significantly, whereas other GTPases purify in the GDP-bound inactive state, SRβ purified in the GTP-bound active state. Furthermore, SRβ bound specifically to ribosomes, but ribosome-binding did not influence the activity of SRβ. The recently solved structure of the SRα-SRβ complex from yeast allows for the discussion of my results from a structural perspective. The results of this work in combination with the published literature allow me to update the existing model for protein targeting and translocation in higher eukaryotes. In this new model SRβ plays a greater role than previously appreciated.</p>