title: Concerted complex assembly and GTPase activation in the chloroplast signal recognition particle.
abstract: The universally conserved signal recognition particle (SRP) and SRP receptor (SR) mediate the cotranslational targeting of proteins to cellular membranes. In contrast, a unique chloroplast SRP in green plants is primarily dedicated to the post-translational targeting of light harvesting chlorophyll a/b binding (LHC) proteins. In both pathways, dimerization and activation between the SRP and SR GTPases mediate the delivery of cargo; whether and how the GTPase cycle in each system adapts to its distinct substrate proteins were unclear. Here, we show that interactions at the active site essential for GTPase activation in the chloroplast SRP and SR play key roles in the assembly of the GTPase complex. In contrast to their cytosolic homologues, GTPase activation in the chloroplast SRP-SR complex contributes marginally to the targeting of LHC proteins. These results demonstrate that complex assembly and GTPase activation are highly coupled in the chloroplast SRP and SR and suggest that the chloroplast GTPases may forego the GTPase activation step as a key regulatory point. These features may reflect adaptations of the chloroplast SRP to the delivery of their unique substrate protein.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E. California Blvd, Pasadena, California 91125, USA.
authors.forename: Thang X
authors.initials: TX
authors.lastname: Nguyen
authors.forename: Sowmya
authors.initials: S
authors.lastname: Chandrasekar
authors.forename: Saskia
authors.initials: S
authors.lastname: Neher
authors.forename: Peter
authors.initials: P
authors.lastname: Walter
authors.forename: Shu-ou
authors.initials: SO
authors.lastname: Shan
mesh_terms: D017360:Arabidopsis / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism*; D000077194:Chlorophyll A; D060365:Chloroplast Proteins; D002736:Chloroplasts / Q000378:metabolism*; D020549:GTP Phosphohydrolase Activators / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000235:genetics / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D045342:Light-Harvesting Protein Complexes / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D018000:Receptors, Peptide / Q000235:genetics / Q000378:metabolism*; D018271:Signal Recognition Particle / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D060365:Chloroplast Proteins; D020549:GTP Phosphohydrolase Activators; D045342:Light-Harvesting Protein Complexes; D008565:Membrane Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; C091479:SRP54A protein, Arabidopsis; D018271:Signal Recognition Particle; C513946:cpFtsY protein, Arabidopsis; C036318:signal peptide receptor; D002734:Chlorophyll; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D000077194:Chlorophyll A