title: The GTPase cycle of the chloroplast import receptors Toc33/Toc34: implications from monomeric and dimeric structures.
abstract: Transport of precursor proteins across chloroplast membranes involves the GTPases Toc33/34 and Toc159 at the outer chloroplast envelope. The small GTPase Toc33/34 can homodimerize, but the regulation of this interaction has remained elusive. We show that dimerization is independent of nucleotide loading state, based on crystal structures of dimeric Pisum sativum Toc34 and monomeric Arabidopsis thaliana Toc33. An arginine residue is--in the dimer--positioned to resemble a GAP arginine finger. However, GTPase activation by dimerization is sparse and active site features do not explain catalysis, suggesting that the homodimer requires an additional factor as coGAP. Access to the catalytic center and an unusual switch I movement in the dimeric structure support this finding. Potential binding sites for interactions within the Toc translocon or with precursor proteins can be derived from the structures.
authors.affiliation: Heidelberg University Biochemistry Center, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Koenig
authors.forename: Mislav
authors.initials: M
authors.lastname: Oreb
authors.forename: Anja
authors.initials: A
authors.lastname: Höfle
authors.forename: Sabine
authors.initials: S
authors.lastname: Kaltofen
authors.forename: Karsten
authors.initials: K
authors.lastname: Rippe
authors.forename: Irmgard
authors.initials: I
authors.lastname: Sinning
authors.forename: Enrico
authors.initials: E
authors.lastname: Schleiff
authors.forename: Ivo
authors.initials: I
authors.lastname: Tews
mesh_terms: D000595:Amino Acid Sequence; D029681:Arabidopsis Proteins / Q000737:chemistry*; D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D019281:Dimerization; D020558:GTP Phosphohydrolases / Q000737:chemistry*; D020690:GTPase-Activating Proteins / Q000737:chemistry; D008565:Membrane Proteins / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D018532:Pisum sativum; D010940:Plant Proteins / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid
chemical_list: D029681:Arabidopsis Proteins; D020690:GTPase-Activating Proteins; D008565:Membrane Proteins; D010940:Plant Proteins; C114786:Toc33 protein, Arabidopsis; C112475:Toc34 protein, plant; D020558:GTP Phosphohydrolases