Substrate binding disrupts dimerization and induces nucleotide exchange of the chloroplast GTPase Toc33

Mislav Oreb, Anja Höfle, Patrick Koenig, Maik S. Sommer, Irmgard Sinning, Fei Wang, Ivo Tews, Danny J. Schnell, Enrico Schleiff

Research output: Contribution to journalArticle

24 Scopus citations

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 regulatorymode and its impact on the current models of protein import into the chloroplast.

Original languageEnglish (US)
Pages (from-to)313-319
Number of pages7
JournalBiochemical Journal
Volume436
Issue number2
DOIs
StatePublished - Jun 1 2011

Keywords

  • Dimeric GTPase
  • G-protein
  • GDP-dissociation-inhibitor function (GDI function)
  • Protein translocation
  • Substrate-based regulation

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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