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

Abstract

The Delta pH pathway is one of two systems for protein transport to the thylakoid lumen. It is a novel transport system that requires only the thylakoidal ΔpH to power translocation. Several substrates of the Delta pH pathway, including the intermediate precursor form of OE17 (iOE17) and the truncated precursor form of OE17 (tOE17), were shown to bind to the membrane in the absence of the ΔpH and be transported into the lumen when the ΔpH was restored. Binding occurred without energy or soluble factors, and efficient transport from the bound state (~80-90%) required only the ΔpH. Binding is due to protein-protein interactions because protease pretreatment of thylakoids destroyed their binding capability. Precursors are bound to a specific site on the Delta pH pathway because binding was competed by saturating amounts of Delta pH pathway precursor proteins, but not by a Sec pathway precursor protein. These results suggested that precursor tOE17 binds to components of the Delta pathway translocation machinery. Hcf106 and Tha4 are two components of the Delta pH pathway machinery. Antibodies to Hcf106 or Tha4, when prebound to thylakoids, specifically inhibited precursor transport on the Delta pH pathway. However, only Hcf106 antibodies reduced the level of precursor binding. These results suggest that Hcf106 functions in early steps of the transport process.