THE THYLAKOID TRANSLOCATION OF SUBUNIT-3 OF PHOTOSYSTEM-I, THE PSAF GENE-PRODUCT, DEPENDS ON A BIPARTITE TRANSIT PEPTIDE AND PROCEEDS ALONGAN AZIDE-SENSITIVE PATHWAY

Subunit 3 of photosystem I (PSI-3), the product of the nuclear psaF ge ne, is the docking protein for plastocyanin during photosynthetic elec tron transport in thylakoid membranes and is synthesized in the cytoso l with a transit peptide that resembles structurally the bipartite tar geting signals of hydrophilic, lumenal components such as plastocyanin , In organello import experiments performed with the authentic PSI-3 p recursor and chimeric polypeptides consisting of residue-correct fusio ns of transit peptides and mature proteins derived from different plas tid proteins demonstrate that the PSI-3 transit peptide is indeed capa ble of translocating proteins into the thylakoid lumen and that, conve rsely, mature PSI-3 depends on a bipartite transit peptide for its thy lakoid transfer, Of the three recently described translocation/integra tion pathways for nucleus encoded proteins carrying bipartite transit peptides that are distinct in their physiological requirements and str ictly protein-specific, PSI-3, like plastocyanin and the 33-kDa protei n of the oxygen evolving complex, is translocated by a pathway that in volves 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 prokary ote-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 p recursor and a mature part of an azide-sensitive polypeptide are still translocated in the presence of the inhibitor.