Chloroplast SecY is complexed to SecE and involved in the translocation ofthe 33-kDa but not the 23-kDa subunit of the oxygen-evolving complex

SecY is a component of the protein-conducting channel for protein transport across the cytoplasmic membrane of prokaryotes. It is intimately associate d with a second integral membrane protein, SecE, and together with SecA for ms the minimal core of the preprotein translocase. A chloroplast homologue of SecY (cpSecY has previously been identified and determined to be localiz ed to the thylakoid membrane. In the present work, we demonstrate that a Se cE homologue is localized to the thylakoid membrane, where it forms a compl ex with cpSecY, Digitonin solubilization of thylakoid membranes releases th e SecY/E complex in a 180-kDa form, indicating that other components are pr esent and/or the complex is a higher order oligomer of the cpSecY/E dimer, To test whether cpSecY forms the protein-conducting channel of the thylakoi d membrane, translocation assays were conducted with the SecA-dependent sub strate OE33 and the SecA-independent substrate OE23, in the presence and ab sence of antibodies raised against cpSecY. The antibodies inhibited translo cation of OE33 but not OE23, indicating that cpSecY comprises the protein-c onducting channel used in the SecA-dependent pathway, whereas a distinct pr otein conducting channel is used to translocate OE23.