A plant secretory signal peptide targets to thylakoid membrane plastome-encoded recombinant proteins

The knowledge of the mechanisms regulating foreign protein folding, targeting, and accumulation in plastids is still incomplete. Here we demonstrate that a plant secretory signal peptide (SP) is able to target a plastome-encoded recombinant protein to the thylakoid membrane. A fusion protein (zeolin) with its native SP expressed by tobacco transplastomic plants was directed into the chloroplast thylakoid membranes, whereas the mutant devoid of the SP, ?zeolin, was instead accumulated in the stroma. We also show that zeolin folds in the thylakoid membrane where it accumulates as trimers linked together by interchain disulphide bonds. Disulphide bonds contributes to protein accumulation since zeolin shows a higher accumulation level with respect to the zeolin variant without cysteines. Stromal ?zeolin is accumulated in low amounts in a monomeric form and is not oxidized. Thus post-transcriptional processes seem to regulate zeolin stability and accumulation. The most plausible zeolin targeting mechanism to thylakoid is discussed herein.