Component specificity for the thylakoidal Sec and Delta pH-dependent protein transport pathways

Area of Science:

• Chloroplast biology
• Protein transport mechanisms
• Molecular genetics

Background:

• Prokaryotes and chloroplast thylakoid membranes share conserved protein export pathways: Sec, signal recognition particle (SRP), and Delta pH/Tat.
• Thylakoid membrane components mediating these pathways are largely uncharacterized.

Purpose of the Study:

• To identify and characterize novel components of the Delta pH protein export pathway in thylakoid membranes.
• To biochemically validate the roles of Tha4, Hcf106, and cpSecY in specific protein translocation pathways.

Main Methods:

• Isolation of a cDNA clone for the novel protein Tha4.
• Generation of antibodies against pea Tha4, maize Hcf106, and cpSecY.
• Localization of proteins to nonappressed thylakoid membranes.
• Inhibition assays using antibodies to assess effects on protein translocation pathways (Delta pH, Sec, SRP).

Main Results:

• Tha4 and Hcf106 were localized to nonappressed thylakoid membranes and found in excess over active translocation sites.
• Antibodies against Tha4 or Hcf106 specifically inhibited Delta pH pathway substrate translocation.
• Antibodies against cpSecY specifically inhibited Sec pathway substrate translocation.
• Tha4 and Hcf106 likely function together in the Delta pH pathway, while cpSecY is specific to the Sec pathway.

Conclusions:

• Tha4 and Hcf106 are confirmed as specific components of the thylakoid Delta pH protein export pathway.
• cpSecY is biochemically validated as a component of the Sec pathway in thylakoid membranes.
• These findings elucidate the molecular machinery of conserved protein export systems in chloroplasts.