Functional analysis of the two Arabidopsis homologues of Toc34, a component of the chloroplast protein import apparatus

Two Arabidopsis Toc34 homologues, atToc34 and atToc33, components of the ch loroplast protein import machinery located in the outer envelope membrane, were recently isolated. Both proteins insert into the outer envelope, are s upposed to bind GTP and to interact with Toc75 as demonstrated by in vitro import assays. We studied the expression of the two genes by RNA gel blot a nalysis, promoter-GUS plants and in situ hybridisations as well as immunobl ot analysis. The atToc34 and atToc33 genes are expressed in green as well a s non-green tissues and are developmentally regulated. Despite these simila rities, however, the two Arabidopsis Toc34 homologues are differentially ex pressed in various plant organs. To gain more insight into the in vivo func tion of both proteins, antisense plants were created. While antisense plant s of atToc33 are characterized by a pale yellowish phenotype, antisense pla nts of atToc34 show a weaker phenotype. Protein interaction studies using a n in vitro translated precursor protein and heterologously expressed atToc3 4 and atToc33 proteins showed a direct GTP-dependent interaction, but demon strated different affinities of the two atToc proteins towards the precurso r protein. Thus, our results indicate a more specialized function for both atToc34 and atToc33, suggesting specificity for certain imported precursor proteins.