title: Multiple conformational switches in a GTPase complex control co-translational protein targeting.
abstract: The "GTPase switch" paradigm, in which a GTPase switches between an active, GTP-bound state and an inactive, GDP-bound state through the recruitment of nucleotide exchange factors (GEFs) or GTPase activating proteins (GAPs), has been used to interpret the regulatory mechanism of many GTPases. A notable exception to this paradigm is provided by two GTPases in the signal recognition particle (SRP) and the SRP receptor (SR) that control the co-translational targeting of proteins to cellular membranes. Instead of the classical "GTPase switch," both the SRP and SR undergo a series of discrete conformational rearrangements during their interaction with one another, culminating in their reciprocal GTPase activation. Here, we show that this series of rearrangements during SRP-SR binding and activation provide important control points to drive and regulate protein targeting. Using real-time fluorescence, we showed that the cargo for SRP--ribosomes translating nascent polypeptides with signal sequences--accelerates SRP.SR complex assembly over 100-fold, thereby driving rapid delivery of cargo to the membrane. A series of subsequent rearrangements in the SRP x SR GTPase complex provide important driving forces to unload the cargo during late stages of protein targeting. Further, the cargo delays GTPase activation in the SRP.SR complex by 8-12 fold, creating an important time window that could further improve the efficiency and fidelity of protein targeting. Thus, the SRP and SR GTPases, without recruiting external regulatory factors, constitute a self-sufficient system that provides exquisite spatial and temporal control of a complex cellular process.
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: Christiane
authors.initials: C
authors.lastname: Schaffitzel
authors.forename: Nenad
authors.initials: N
authors.lastname: Ban
authors.forename: Shu-ou
authors.initials: SO
authors.lastname: Shan
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000378:metabolism / Q000502:physiology*; D020558:GTP Phosphohydrolases / Q000378:metabolism / Q000502:physiology*; D046912:Multiprotein Complexes; D011485:Protein Binding; D011487:Protein Conformation; D021381:Protein Transport; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology*; D018000:Receptors, Peptide / Q000737:chemistry*; D012270:Ribosomes / Q000378:metabolism; D018271:Signal Recognition Particle / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology
chemical_list: D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; C082931:Ffh protein, E coli; C087406:FtsY protein, Bacteria; D046912:Multiprotein Complexes; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; D018271:Signal Recognition Particle; C036318:signal peptide receptor; D020558:GTP Phosphohydrolases