A reversal of Tat

Nearly 15 years of in vitro experiments showed that the Tat transport system, which spans the thylakoid membrane in chloroplasts, sends proteins unidirectionally inwards, from stroma to lumen. On page 281, Di Cola and Robinson report in vivo evidence from protoplasts showing that a large fraction of the proteins that move part way through the channel end up back in the stroma.

The Tat system is required for transporting several protein components of a photosynthetic complex to the lumen of the chloroplast. For example, the 23K protein starts out in the cytoplasm as a 33-kD precursor, containing two NH₂-terminal signal sequences. The first signal sequence is cleaved as the protein moves across the outer chloroplast membrane into the stroma, and the second as the protein passes through the Tat transport...