Members of the Toc159 import receptor family represent distinct pathways for protein targeting to plastids

Yordanka Ivanova¹, Matthew D. Smith¹, Kunhua Chen², and Danny J. Schnell¹^*

¹ Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, MA 01003, USA
² Plant Molecular and Cellular Biology Laboratory, The Salk Institute, La Jolla, CA 92037, USA

^* Corresponding author. E-mail address: dschnell{at}biochem.umass.edu.

Plastids represent a diverse group of organelles that performessential metabolic and signaling functions within all plantcells. The differentiation of specific plastid types relieson the import of selective sets of proteins from among the $~$ 2500nucleus-encoded plastid proteins. The Toc159 family of GTPasesmediates the initial targeting of proteins to plastids. In Arabidopsisthaliana, the Toc159 family consists of four genes: atTOC159,atTOC132, atTOC120, and atTOC90. In vivo analysis of atToc159function indicates that it is required specifically for theimport of proteins necessary for chloroplast biogenesis (Baueret al., 2000, Nature, 403:203-7). In this report, we demonstratethat atToc120 and atToc132 represent a structurally and functionallyunique subclass of protein import receptors. Unlike atToc159,mutants lacking both atToc120 and atToc132 are inviable. Furthermore,atToc120 and atToc132 exhibit preprotein binding propertiesthat are distinct from atToc159. These data indicate that thedifferent members of the Toc159 family represent distinct pathwaysfor protein targeting to plastids, and are consistent with thehypothesis that separate pathways have evolved to ensure balancedimport of essential proteins during plastid development.