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

Prokaryotes and prokaryote-derived thylakoid membranes of chloroplasts shar e multiple, evolutionarily conserved pathways for protein export. These inc lude the Sec, signal recognition particle (SRP), and Delta pH/Tat systems. Little is known regarding the thylakoid membrane components involved in the se pathways. We isolated a cDNA clone to a navel component of the Delta pH pathway, Tha4, and pre pared antibodies against pea Tha4, against maize Hcf 106, a protein implicated in Delta pH pathway transport by genetic studies, and against cpSecY, the thylakoid homologue of the bacterial SecY transloc on protein. These components were localized to the non-appressed thylakoid membranes. Tha4 and Hcf106 were present in similar to 10-fold excess over a ctive translocation sites. Antibodies to either Tha4 or Hcf106 inhibited tr anslocation of four known Delta pH pathway substrate proteins, but not of S ec pathway or SRP pathway substrates. This suggests that Tha4 and Hcf106 op erate either in series or as subunits of a heteromultimeric complex. cpSecY antibodies inhibited translocation of Sec pathway substrates but not of De lta pH or SRP pathway substrates. These studies provide the first biochemic al evidence that Tha4 and Hcf106 are specific components of the Delta pH pa thway and provide one line of evidence that cpSecY is used specifically by the Sec pathway.