Translocation of proteins across membranes is essential for the biogenesis of each cell and is achieved by proteinaceous complexes. We analyzed the translocation complex of the intermembrane space from chloroplasts and identified a 12-kDa protein associated with the Toc machinery. Toc12 is an outer envelope protein exposing a soluble domain into the intermembrane space. Toc12 contains a J-domain and stimulates the ATPase activity of DnaK. The conformational stability and the ability to stimulate Hsp70 are dependent on a disulfide bridge within the loop region of the J-domain, suggesting a redox-regulated activation of the chaperone. Toc12 is associated with Toc64 and Tic22. Its J-domain recruits the Hsp70 of outer envelope membrane to the intermembrane space translocon and facilitates its interaction to the preprotein.

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Phytochromes are light responsive photoreceptors in plants that influence development and differentiation during the entire plant life cycle. Plant nucleoside diphosphate kinase 2 (NDPK2) has been reported to be a component of the light-mediated signalling cascade and to interact physically with phytochrome A in the cytosol. By using diverse methods as in vitro imports, in vivo localisation of GFP-fusion proteins and immuno blotting of plant cell fractions we clearly localise NDPK2 only to chloroplasts but not to the cytosol, demonstrating that although high affinity protein-protein interactions can occur in vitro, their physiological relevance can be artificial if the proteins are localised to different cell compartments in vivo.

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