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© The Rockefeller University Press, 0021-9525/1999//45 $5.00
The Journal of Cell Biology, Volume 146, Number 1, , 1999 45-56

Original Article

Component Specificity for the Thylakoidal Sec and Delta Ph–Dependent Protein Transport Pathways



Hiroki Moria, Elizabeth J. Summera, Xianyue Maa, and Kenneth Clinea

a Horticultural Sciences and Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, Florida 32611
Horticultural Sciences Department, Fifield Hall, University of Florida, Gainesville, FL 32611.(352) 392-5653(352) 392-4711 x 219

kcline{at}ufl.edu

Prokaryotes and prokaryote-derived thylakoid membranes of chloroplasts share multiple, evolutionarily conserved pathways for protein export. These include the Sec, signal recognition particle (SRP), and Delta pH/Tat systems. Little is known regarding the thylakoid membrane components involved in these pathways. We isolated a cDNA clone to a novel component of the Delta pH pathway, Tha4, and prepared antibodies against pea Tha4, against maize Hcf106, a protein implicated in Delta pH pathway transport by genetic studies, and against cpSecY, the thylakoid homologue of the bacterial SecY translocon protein. These components were localized to the nonappressed thylakoid membranes. Tha4 and Hcf106 were present in ~10-fold excess over active translocation sites. Antibodies to either Tha4 or Hcf106 inhibited translocation of four known Delta pH pathway substrate proteins, but not of Sec pathway or SRP pathway substrates. This suggests that Tha4 and Hcf106 operate either in series or as subunits of a heteromultimeric complex. cpSecY antibodies inhibited translocation of Sec pathway substrates but not of Delta pH or SRP pathway substrates. These studies provide the first biochemical evidence that Tha4 and Hcf106 are specific components of the Delta pH pathway and provide one line of evidence that cpSecY is used specifically by the Sec pathway.

Key Words: chloroplast protein transport • twin arginine • SecY • Hcf106 • Tha4

© 1999 The Rockefeller University Press

1.used in this paper: i, intermediate precursor form; LHCP, light-harvesting chlorophyll a/b protein; m, mature form; OE33, OE23, and OE17, 33-, 23-, and 17-kD subunits of the photosystem II oxygen-evolving complex; p, precursor form; PC, plastocyanin; PSI-N, N subunit of the photosystem I complex; PSII-T, T subunit of the photosystem II complex; SRP, signal recognition particle


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