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Horticultural Sciences Department and Plant Molecular and Cellular Biology Program, University of Florida, Gainesville,
Florida 32611
Transport of proteins to the thylakoid lumen
is accomplished by two precursor-specific pathways, the
Sec and the unique Delta pH transport systems. Pathway selection is specified by transient lumen-targeting
domains (LTDs) on precursor proteins. Here, chimeric and mutant LTDs were used to identify elements responsible for targeting specificity. The results showed
that: (a) minimal signal peptide motifs consisting of
charged N, hydrophobic H, and cleavage C domains
were both necessary and sufficient for pathway-specific targeting; (b) exclusive targeting to the Delta pH pathway requires a twin arginine in the N domain and an H
domain that is incompatible with the Sec pathway; (c)
exclusive targeting to the Sec pathway is achieved by an
N domain that lacks the twin arginine, although the
twin arginine was completely compatible with the Sec
system. A dual-targeting signal peptide, constructed by
combining Delta pH and Sec domains, was used to simultaneously compare the transport capability of both
pathways when confronted with different passenger proteins. Whereas Sec passengers were efficiently
transported by both pathways, Delta pH passengers
were arrested in translocation on the Sec pathway. This
finding suggests that the Delta pH mechanism evolved
to accommodate transport of proteins incompatible
with the thylakoid Sec machinery.
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