title: Substrate binding disrupts dimerization and induces nucleotide exchange of the chloroplast GTPase Toc33.
abstract: GTPases act as molecular switches to control many cellular processes, including signalling, protein translation and targeting. Switch activity can be regulated by external effector proteins or intrinsic properties, such as dimerization. The recognition and translocation of pre-proteins into chloroplasts [via the TOC/TIC (translocator at the outer envelope membrane of chloroplasts/inner envelope membrane of chloroplasts)] is controlled by two homologous receptor GTPases, Toc33 and Toc159, whose reversible dimerization is proposed to regulate translocation of incoming proteins in a GTP-dependent manner. Toc33 is a homodimerizing GTPase. Functional analysis suggests that homodimerization is a key step in the translocation process, the molecular functions of which, as well as the elements regulating this event, are largely unknown. In the present study, we show that homodimerization reduces the rate of nucleotide exchange, which is consistent with the observed orientation of the monomers in the crystal structure. Pre-protein binding induces a dissociation of the Toc33 homodimer and results in the exchange of GDP for GTP. Thus homodimerization does not serve to activate the GTPase activity as discussed many times previously, but to control the nucleotide-loading state. We discuss this novel regulatory mode and its impact on the current models of protein import into the chloroplast.
authors: Center for Membrane Proteomics and the Cluster of Excellence Frankfurt, Department of Biosciences, Molecular Cell Biology of Plants, Goethe University Frankfurt, Max-von-Laue Str. 8, D-60438 Frankfurt am Main, Germany.
authors: Mislav
authors: M
authors: Oreb
authors: Anja
authors: A
authors: Höfle
authors: Patrick
authors: P
authors: Koenig
authors: Maik S
authors: MS
authors: Sommer
authors: Irmgard
authors: I
authors: Sinning
authors: Fei
authors: F
authors: Wang
authors: Ivo
authors: I
authors: Tews
authors: Danny J
authors: DJ
authors: Schnell
authors: Enrico
authors: E
authors: Schleiff
mesh_terms: D000595:Amino Acid Sequence; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002736:Chloroplasts / Q000201:enzymology* / Q000235:genetics; D020558:GTP Phosphohydrolases / Q000235:genetics / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000235:genetics / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D011485:Protein Binding / Q000235:genetics; D055503:Protein Multimerization / Q000235:genetics / Q000502:physiology*; D011498:Protein Precursors / Q000378:metabolism; D013379:Substrate Specificity / Q000235:genetics
chemical_list: D029681:Arabidopsis Proteins; D008565:Membrane Proteins; D011498:Protein Precursors; C114786:Toc33 protein, Arabidopsis; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases