Poster: Protein targeting & vesicular trafficking

Abs # 657: atToc159 is a selective transit peptide receptor for the import of nucleus-encoded chloroplast proteins

Presenter:	Rounds, Caleb M, Caleb_rounds@hotmail.com
Authors	Rounds, Caleb M (A)   Smith, Matt D (A)   Wang, Fei  (A)   Schnell, Danny J (A)
Affiliations:	(A): University of Massachusetts

The members of the Toc159 family of GTPases have been implicated as the primary receptors for import of nucleus-encoded preproteins into plastids. The most abundant member of this family in chloroplasts, Toc159, distributes between a soluble cytoplasmic form and a chloroplast-bound form and has been shown to covalently cross-link to bound preproteins at the chloroplast surface. An Arabidopsis Toc159 null mutant, ppi2, is deficient in chloroplast-specific photosynthetic proteins, resulting in the inability of proplastids to develop into chloroplasts during photomorphogenesis. These studies led to the hypothesis that Toc159 functions as a selective cytoplasmic import receptor for preproteins that are required for chloroplast development. In this report, we provide evidence that the ppi2 phenotype results directly from a defect in the import of several highly expressed photosynthetic preproteins. Furthermore, we demonstrate that the cytoplasmic and recombinant forms of soluble Toc159 bind directly and selectively to the transit peptides of these representative photosynthetic preproteins, but not representative constitutively-expressed plastid preproteins. These data support the function of Toc159 as a cytoplasmic import receptor that defines a selective pathway for the targeting of a set of preproteins required for chloroplast biogenesis.