title: Demonstration of a multistep mechanism for assembly of the SRP x SRP receptor complex: implications for the catalytic role of SRP RNA.
abstract: Two GTPases in the signal recognition particle (SRP) and its receptor (SR) control the delivery of newly synthesized proteins to the endoplasmic reticulum or plasma membrane. During the protein targeting reaction, the 4.5S SRP RNA accelerates the association between the two GTPases by 400-fold. Using fluorescence resonance energy transfer, we demonstrate here that formation of a stable SRP x SR complex involves two distinct steps: a fast initial association between SRP and SR to form a GTP-independent early complex and then a GTP-dependent conformational rearrangement to form the stable final complex. We also found that the 4.5S SRP RNA significantly stabilizes the early GTP-independent intermediate. Furthermore, mutational analyses show that there is a strong correlation between the ability of the mutant SRP RNAs to stabilize the early intermediate and their ability to accelerate SRP x SR complex formation. We propose that the SRP RNA, by stabilizing the early intermediate, can give this transient intermediate a longer life time and therefore a higher probability to rearrange to the stable final complex. This provides a coherent model that explains how the 4.5S RNA exerts its catalytic role in SRP x SR complex assembly.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Zhang
authors.forename: Simon
authors.initials: S
authors.lastname: Kung
authors.forename: Shu-ou
authors.initials: SO
authors.lastname: Shan
mesh_terms: D002384:Catalysis; D004721:Endoplasmic Reticulum / Q000502:physiology; D029968:Escherichia coli Proteins / Q000737:chemistry; D031541:Fluorescence Resonance Energy Transfer; D020558:GTP Phosphohydrolases / Q000502:physiology; D009154:Mutation; D012329:RNA, Bacterial; D012335:RNA, Ribosomal / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry* / Q000502:physiology; D018000:Receptors, Peptide / Q000737:chemistry* / Q000502:physiology; D018271:Signal Recognition Particle / Q000737:chemistry* / Q000235:genetics / Q000502:physiology
chemical_list: C409968:4.5S RNA; D029968:Escherichia coli Proteins; D012329:RNA, Bacterial; D012335:RNA, Ribosomal; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; D018271:Signal Recognition Particle; C036318:signal peptide receptor; D020558:GTP Phosphohydrolases