The thylakoid translocation of subunit 3 of photosystem I, the psaF gene product, depends on a bipartite transit peptide and proceeds along an azide-sensitive pathway.

Subunit 3 of photosystem I (PSI-3), the product of the nuclear psaF gene, is the docking protein for plastocyanin during photosynthetic electron transport in thylakoid membranes and is synthesized in the cytosol with a transit peptide that resembles structurally the bipartite targeting signals of hydrophilic, lumenal components such as plastocyanin. In organello import experiments performed with the authentic PSI-3 precursor and chimeric polypeptides consisting of residue-correct fusions of transit peptides and mature proteins derived from different plastid proteins demonstrate that the PSI-3 transit peptide is indeed capable of translocating proteins into the thylakoid lumen and that, conversely, mature PSI-3 depends on a bipartite transit peptide for its thylakoid transfer. Of the three recently described translocation/integration pathways for nucleus-encoded proteins carrying bipartite transit peptides that are distinct in their physiological requirements and strictly protein-specific, PSI-3, like plastocyanin and the 33-kDa protein of the oxygen-evolving complex, is translocated by a pathway that involves stromal factors but no proton gradient across the membrane. It is not affected by saturating amounts of the precursor for the 23-kDa protein of the oxygen-evolving complex that follows the latter route. Thylakoid translocation of PSI-3 is, however, impaired in the presence of sodium azide, which indicates that a homolog to the bacterial SecA protein might be involved in this process suggesting, thus, a prokaryote-like translocation pathway. The azide-sensitive factor appears to interact predominantly with the transit peptide of a precursor protein, since chimeras consisting of a presequence from an azide-resistant precursor and a mature part of an azide-sensitive polypeptide are still translocated in the presence of the inhibitor.