An essential role of a TatC homologue of a Delta pH-dependent protein transporter in thylakoid membrane formation during chloroplast development in Arabidopsis thaliana

At least three transport systems function in targeting nuclear-encoded chlo roplast proteins to the chloroplast thylakoid membrane. one of these system s requires a thylakoid pH gradient and is named the Delta pH-dependent prot ein transport system. A similar Delta pH export system of Escherichia coli contains four components, twin arginine translocation A (TatA), TatB, TatC, and TatE. TatC is a major component of the Delta pH-dependent protein tran sporter in E. coli and functions In the translocation of tightly folded pro teins across membranes. We have isolated four transposon-inserted albino mu tants named albino and pale green 2 (apg2) from Arabidopsis thaliana and sh owed that the transposons were inserted into different sites of a single ge ne. The APG2 gene product(named cpTatC) has sequence similarity with bacter ial TatC and contains six putative transmembrane domains, including bacteri al TatC proteins and a transit peptide in its N terminus. apg2 mutants show ed albino phenotypes and could not grow in soil. The apg2 plastids were hig hly vacuolated, lacked internal membrane structures and lamellae of the thy lakoid membrane, and contained many densely stained globule structures, lik e undifferentiated proplastids. Immunoblot analysis detected no thylakoid m embrane proteins such as D1, light-harvesting complex, and OE23 in apg2 pla stids, whereas soluble proteins such as rubisco large and small subunits we re not decreased. These results indicate an essential role of cpTatC in chl oroplast development, especially in thylakoid membrane formation.