Srp

Chloroplast SRP receptor

Mature plastocyanin

Mature metal-binding protein

Chloroplast SRP receptor

M FIGURE 16-31 Two of the four pathways for transporting proteins from the cytosol to the thylakoid lumen. In these pathways, unfolded precursors are delivered to the stroma via the same outer-membrane proteins that import stromal-localized proteins. Cleavage of the N-terminal stromal-import sequence by a stromal protease then reveals the thylakoid-targeting sequence. At this point the two pathways diverge. In the SRP-dependent pathway (left), plastocyanin and similar proteins are kept unfolded in the stromal space by a set of chaperones (not shown) and, directed by the thylakoid-targeting sequence, bind to proteins that are closely related to the bacterial SRP SRP receptor, and SecY translocon, which mediate movement into the lumen. After the thylakoid-targeting sequence is removed in the thylakoid lumen by a separate endoprotease, the protein folds into its mature conformation. In the pH-dependent pathway (right), metal-binding proteins fold in the stroma, and complex redox cofactors are added. Two arginine residues (RR) at the N-terminus of the thylakoid-targeting sequence and a pH gradient across the inner membrane are required for transport of the folded protein into the thylakoid lumen. The translocon in the thylakoid membrane is composed of at least four proteins related to proteins in the bacterial inner membrane. [See R. Dalbey and C. Robinson, 1999, Trends Biochem. Sci. 24:17; R. E. Dalbey and A. Kuhn, 2000, Ann. Rev. Cell Devel. Biol. 16:51; and C. Robinson and A. Bolhuis, 2001, Nature Rev. Mol. Cell Biol. 2:350.]

Mature plastocyanin

Mature metal-binding protein utilizes a protein related to bacterial SecA and is thought to utilize a mechanism similar to that depicted in Figure 16-23. A third pathway, which targets proteins to the thylakoid membrane, depends on a protein related to the mitochon-drial Oxa1 protein and the homologous bacterial protein (see Figure 16-29, path B). Some proteins encoded by chloroplast DNA and synthesized in the stroma or transported into the stroma from the cytosol are inserted into the thylakoid membrane via this pathway.

Finally, thylakoid proteins that bind metal-containing co-factors follow another pathway into the thylakoid lumen (Figure 16-31, ApH pathway). The unfolded precursors of these proteins are first targeted to the stroma, where the N-

terminal stromal-import sequence is cleaved off and the protein then folds and binds its cofactor. A set of thylakoid-membrane proteins assists in translocating the folded protein and bound cofactor into the thylakoid lumen, a process powered by the pH gradient normally maintained across the thylakoid membrane. The thylakoid-targeting sequence that directs a protein to this pH-dependent pathway includes two closely spaced arginine residues that are crucial for recognition. Bacterial cells also have a mechanism for translocating folded proteins with a similar arginine-containing sequence across the inner membrane. The molecular mechanism whereby these large folded globular proteins are transported across the thylakoid membrane is currently under intense study.

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