von Loeffelholz O, Kriechbaumer V, Ewan RA, Jonczyk R, Lehmann S, Young JC, Abell BM

Chloroplast precursor proteins encoded in the nucleus depend on their targeting sequences for delivery to chloroplasts. There exist different routes to the chloroplast outer envelope, but a common theme is the involvement of molecular chaperones. Hsp90 delivers precursors via its receptor Toc64, which transfers precursors to the core translocase in ...

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the outer envelope. Here, we identify an uncharacterised protein in Arabidopsis thaliana, OEP61, which shares common features with Toc64, and potentially provides an alternative route to chloroplasts. Sequence analysis indicates that OEP61 possesses a clamp-type tetratricopeptide repeat (TPR) domain capable of binding molecular chaperones, and a C-terminal transmembrane domain. Phylogenetic comparisons show sequence similarities between the TPR domain of OEP61 and those of the Toc64 family. Expression of mRNA and protein was detected in all plant tissues, and localisation at the chloroplast outer envelope was demonstrated by a combination of microscopy and in vitro import assays. Binding assays show that OEP61 interacts specifically with Hsp70 via its TPR clamp domain. Furthermore, OEP61 selectively recognises chloroplast precursors via their targeting sequences, and a soluble form of OEP61 inhibits chloroplast targeting. We therefore propose that OEP61 is a novel chaperone receptor at the chloroplast outer envelope, mediating Hsp70-dependent protein targeting to chloroplasts.

Date: May. 25, 2011

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