The thylakoidal Tat receptor complex appears as a homo-trimeric TatC core with three associated TatB subunits

The Twin-arginine translocation (Tat) machinery, which is found in most cellular membranes containing a respiratory or photosynthetic electron transport chain, is characterized by its unique ability to catalyze membrane transport of folded proteins without impairing the membrane potential. In plant thylakoids, Tat machinery consists of three subunits, TatA, TatB, and TatC, with the latter two, TatB and TatC, forming membrane-integral multimeric TatBC receptor complexes. Here we have analyzed the stability and the subunit composition of these complexes after solubilization of thylakoids with the mild detergent digitonin as well as after additional affinity-purification. Employing different detergent combinations and/or heat treatment (40 °C) followed by BN-PAGE and Western analysis we could identify four distinct Tat complexes with apparent molecular masses ranging from approximately 230 kDa to 620 kDa. Treatment of the largest Tat complex with either heat or detergents like DDM or Triton X-114 led to its stepwise breakdown into the three smaller complexes resulting from the successive release of TatB subunits from a relatively stable TatC core complex. From these data we postulate that the fully assembled, physiologically active TatBC receptor complex consists of a stable, trimeric TatC core to which three TatB subunits are bound independently from each other.