Dual targeting of TatA points to a chloroplast-like Tat pathway in plant mitochondria

The biogenesis of membrane-bound electron transport chains requires membrane translocation pathways for folded proteins carrying complex cofactors, like the Rieske Fe/S proteins. Two independent systems were developed during evolution, namely the Twin-arginine translocation (Tat) pathway, which is present in bacteria and chloroplasts, and the Bcs1 pathway found in mitochondria of yeast and mammals. Mitochondria of plants carry a Tat-like pathway which was hypothesized to operate with only two subunits, a TatB-like protein and a TatC homolog (OrfX), but lacking TatA. Here we show that the nuclearly encoded TatA from pea has dual targeting properties, i.e., it can be imported into both, chloroplasts and mitochondria. Dual targeting of TatA was observed with in organello experiments employing chloroplasts and mitochondria isolated from pea as well as after transient expression of suitable reporter constructs in leaf tissue from pea and Nicotiana benthamiana. The extent of transport of these constructs into mitochondria of transiently transformed leaf cells was relatively low, causing a demand for highly sensitive methods to be detected, like the sasplitGFP approach. Yet, the dual import of TatA into mitochondria and chloroplasts observed here points to a common mechanism of Tat transport for folded proteins within both endosymbiotic organelles in plants.