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© The Rockefeller University Press, 0021-9525/1997//823 $5.00
The Journal of Cell Biology, Volume 136, Number 4, , 1997 823-832

Article

Targeting Determinants and Proposed Evolutionary Basis for the Sec and the Delta pH Protein Transport Systems in Chloroplast Thylakoid Membranes



Ralph Henry, Matthew Carrigan, Michael McCaffery, Xianyue Ma, and Kenneth Cline

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.

Abbreviations used in this paper: DT, dual-targeting; LTD, lumen-targeting domain; PC, plastocyanin; SE, stromal extract; SOE, splicing by overlap extension; STD, stroma-targeting domain.

Address all correspondence to Kenneth Cline, Horticultural Sciences Department, Fifield Hall, University of Florida, Gainsville, FL 32611. Tel.: (352) 392-4711 ext. 219. Fax: (352) 392-5653. e-mail: KCC{at}icbr.ifas.ufl.edu


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