Two Toc34 homologues with different properties.

The Toc34 isoforms are located in the outer envelope membrane of plastids. In pea, Toc34 functions as a GTP dependent receptor for preproteins, which is controlled by protein phosphorylation. Two members of this family are present in Arabidopsis thaliana, namely, atToc34 and atToc33. AtToc33 is phosphorylated, as is the homologue in P. sativum, while atToc34 is not. The phosphorylation of atToc33 occurs on serine 181. The highest affinity for dimerization was for the heterodimer between Toc33 and Toc34 in the absence of GTP or GDP. Both proteins, atToc33 and atToc34, bind GTP with significantly higher affinity than GDP and are able to hydrolyze GTP. The intrinsic GTP hydrolysis rate of both proteins is comparable. Hydrolysis is strongly stimulated in the presence of preproteins, which are in turn released upon GTP hydrolysis. Preprotein subclasses exist, which show a strong preference for either the atToc33 or the atToc34 receptor as revealed by GTP hydrolysis rate stimulation and receptor precursor dissociation constants. Detailed analysis of precursor recognition supports the model of a GTP hydrolysis regulated receptor ligand interaction.