TRANSPORT OF PROTEINS TOWARDS THE CHLOROPLAST THYLAKOID LUMEN

ABSTRACT

Many proteins found in the chloroplast are synthesized in the cytoplasm as transit peptide-containing precursor molecules. Stroma-targeted proteins have to cross the two envelope membranes; lumen proteins, however, have also to be transferred over the thylakoid membrane, i.e., they have to cross three biological membranes in order to reach their final location. Recent evidence shows that the routing of plastocyanin (PC) towards the lumen involves two post-translational transport processes and is mediated by two different regions of the transit peptide. Concomitant with the transport, the transit peptide is removed in two successive steps. The transit peptide consists of two functionally different domains. In this study we examine to what extent each domain is involved in import and routing and how far these two processes are linked. The results show that the N-terminal part of the transit peptide is responsible for import into the chloroplast. The N-terminal 43 amino acids are sufficient to direct other proteins into the stroma. The C-terminal part of the transit peptide is a prerequisite for routing inside the chloroplast but not for import. Transgenic tomato plants that constitutively express a foreign PC gene were used to study protein transport into different tissues. We conclude from these experiments that plastids of developmentally different tissues are capable of importing precursor proteins normally not found in these tissues. Most likely such plastids, though functionally and morphologically differentiated, have similar or identical protein import mechanisms when compared to the chloroplasts in green tissue. It is postulated that this transport mechanism evolved by the addition of a chloroplast-specific targeting peptide to the signal peptide-containing PC precursor of the endosymbiont, after the genetic information for this thylakoid protein had been transferred from the endosymbiont to the nucleus.