Precursors bind to specific sites on thylakoid membranes prior to transport on the delta pH protein translocation system

The Delta pH pathway is one of two systems for protein transport to the thy lakoid lumen. it is a novel transport system that requires only the thylako idal Delta pH to power translocation, Several substrates of the Delta pH pa thway, including the intermediate precursor form of OE17 (iOE17) and the tr uncated precursor form of OE17 (tOE17), were shown to bind to the membrane in the absence of the Delta pH and be transported into the lumen when the D elta pH was restored. finding occurred without energy or soluble factors, a nd efficient transport from the bound state (similar to 80-90%) required on ly the Delta pH, Binding is due to protein-protein interactions because pro tease pretreatment of thylakoids destroyed their binding capability, Precur sors are bound to a specific site on the Delta pH pathway because binding w as competed by saturating amounts of Delta pH pathway precursor proteins, b ut not by a Sec pathway precursor protein, These results suggested that pre cursor tOE17 binds to components of the Delta pathway translocation machine ry. Hcf106 and Thai are two components of the Delta pH pathway machinery, A ntibodies to Hcf106 or Tha4, when prebound to thylakoids, specifically inhi bited precursor transport on the Delta pH pathway. However, only Hcf106 ant ibodies reduced the level of precursor binding. These results suggest that Hcf106 functions in early steps of the transport process.