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Published online 13 August 2001. doi:10.1083/jcb.200105149
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© The Rockefeller University Press, 0021-9525/2001/8/719 $5.00
The Journal of Cell Biology, Volume 154, Number 4, August 20, 2001 719-730

Article

Thylakoid {Delta}pH-dependent precursor proteins bind to a cpTatC–Hcf106 complex before Tha4-dependent transport



Kenneth Cline and Hiroki Mori

Horticultural Sciences and Plant Molecular and Cellular Biology, University of Florida, Gainesville, FL 32611

Address correspondence to Kenneth Cline, Horticultural Sciences Department, Fifield Hall, University of Florida, Gainesville, Florida 32611. Tel.: (352) 392-4711 ext. 219. Fax: (352) 392-5653. E-mail: kcline{at}ufl.edu

The thylakoid {Delta}pH-dependent pathway transports folded proteins with twin arginine–containing signal peptides. Identified components of the machinery include cpTatC, Hcf106, and Tha4. The reaction occurs in two steps: precursor binding to the machinery, and transport across the membrane. Here, we show that a cpTatC–Hcf106 complex serves as receptor for specific binding of twin arginine–containing precursors. Antibodies to either Hcf106 or cpTatC, but not Tha4, inhibited precursor binding. Blue native gel electrophoresis and coimmunoprecipitation of digitonin-solubilized thylakoids showed that Hcf106 and cpTatC are members of an ~700-kD complex that lacks Tha4. Thylakoid-bound precursor proteins were also associated with an ~700-kD complex and were coimmunoprecipitated with antibodies to cpTatC or Hcf106. Chemical cross-linking revealed that precursors make direct contact with cpTatC and Hcf106 and confirmed that Tha4 is not associated with precursor, cpTatC, or Hcf106 in the membrane. Precursor binding to the cpTatC–Hcf106 complex required both the twin arginine and the hydrophobic core of the signal peptide. Precursors remained bound to the complex when Tha4 was sequestered by antibody, even in the presence of {Delta}pH. These results indicate that precursor binding to the cpTatC–Hcf106 complex constitutes the recognition event for this pathway and that subsequent participation by Tha4 leads to translocation.

Key Words: chloroplast; receptor; Tat pathway, signal peptide; twin arginine


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