Evidence for a loop mechanism of protein transport by the thylakoid Delta pH pathway

The thylakoid Delta pH pathway is a protein transport system with unprecedented characteristics. To investigate its mechanism, the topology of precursor insertion was determined. A fusion protein comprising a large polypeptide domain fused to the amino terminus of pOE17 (a Delta pH pathway precursor) was efficiently processed by thylakoid membranes. The amino terminus, including the targeting peptide, remained on the cis side of the membrane. Mature OE17 was transported to the lumen. These experiments demonstrate that Delta pH directed precursors enter the thylakoid membrane in a loop, implying that the Delta pH pathway has evolved from an export-type protein translocation system.