Poster: Protein Targeting & Vesicular Trafficking

Abs # 1190: Preprotein binding by the Toc159 receptor, and requirements for targeting the receptor to chloroplasts

Presenter:	Smith, Matthew D, msmith@biochem.umass.edu
Authors	Smith, Matthew D (A)   Rounds, Caleb M (A)   Kessler, Felix  (B)   Schnell, Danny J (A)
Affiliations:	(A): Department of Biochemistry and Molecular Biology, University of Massachusetts (B): Institut de Botanique, Laboratoire de Physiologie Vegetale, Universite de Neuchatel

The Toc complex of the outer chloroplast envelope is responsible for initiating the import of nuclear-encoded preproteins from the cytoplasm into the organelle. The Arabidopsis Toc complex is composed of three major components: atToc33, atToc75 and atToc159. atToc75 comprises at least part of the preprotein channel through the outer membrane, whereas atToc159 and atToc33 are GTPases that function in preprotein recognition and regulation of import, respectively. Recent evidence suggests that atToc159 may act as a soluble preprotein receptor that cycles between the cytoplasm and outer chloroplast membrane. Targeting of the receptor to the chloroplast surface is mediated by its GTPase domain, through an interaction with the homologous GTPase domain of atToc33. The subsequent productive integration of atToc159 into the Toc complex requires its intrinsic GTPase activity. Here, we demonstrate that a functional GTPase domain is required for preprotein binding by atToc159 and that the receptor specifically binds to transit peptides. In addition, the overexpressed GTPase domain of atToc159 can inhibit the import of preproteins into chloroplasts in vitro, suggesting that this domain of the receptor comprises at least part of the preprotein binding site. We will present our most recent data on the mechanism of preprotein binding by the atToc159 receptor and its targeting to the chloroplast.