Targeting of Proteins Into and Across the Thylakoid Membrane
The synthesis and assembly of photosynthetic proteins is a complex process in chloroplasts, since a variety of chloroplast- and nuclear-encoded proteins are targeted either into or across the thylakoid membrane. The insertion of chloroplast proteins is poorly understood, but in vitro reconstitution assays have provided a great deal of information on the pathways taken by imported thylakoid proteins. The import of these proteins from the cytosol is dependent on N-terminal presequences which serve to target the proteins across the double-membrane envelope, probably at contact sites between the two membranes. Most integral membrane proteins are subsequently targeted into the thylakoid by means of information located in the mature proteins, whereas cleavable targeting signals direct the translocation of hydrophilic lumenal proteins across the thylakoid membrane. Remarkably, this transport of lumenal proteins across the thylakoid membrane relies on at least two completely different mechanisms, and the available evidence strongly suggests that distinct translocases operate in the thylakoid membrane. One of these mechanisms appears to have been inherited from the cyanobacterial-type progenitor of the chloroplast, whereas the other may have evolved relatively recently in response to the appearance of novel photosynthetic proteins.
KeywordsThylakoid Membrane Transit Peptide Signal Recognition Particle Envelope Membrane Thylakoid Lumen
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