A SecY homologue is involved in chloroplast-encoded D1 protein biogenesis

We have used the photosystem II reaction center D1 protein as a model to st udy the mechanisms of targeting and insertion of chloroplast-encoded thylak oid membrane proteins. The unusually high turnover rate and distinct pausin g intermediates during translation make the D1 protein biogenesis particula rly suitable for these purposes. Here we show that cpSecY, a chloroplast ho mologue of bacterial essential translocon component SecY, interacts tightly with thylakoid membrane-bound ribosomes, suggesting its involvement in pro tein translocation and insertion. Co-immunoprecipitation and cross-linking experiments indicated that cpSecY resides in the vicinity of D1 elongation intermediates and provided evidence for a transient interaction of cpSecY w ith D1 elongation intermediates during the biogenesis of D1. After terminat ion of translation, such interactions no longer existed. Our results indica te that, in addition to a well characterized role of cpSecY in posttranslat ional translocation of nuclear-encoded proteins, it seems to be also involv ed in cotranslational membrane protein translocation and insertion in chlor oplasts.