Interactions of ribosome nascent chain complexes of the chloroplast-encoded D1 thylakoid membrane protein with cpSRP54

The mechanisms of targeting, insertion and assembly of the chloroplast-encoded thylakoid membrane proteins are unknown. In this study, we investigated these mechanisms for the chloroplast-encoded polytopic D1 thylakoid membrane protein, using a homologous translation system isolated from tobacco chloroplasts. Truncated forms of the psbA gene were translated and stable ribosome nascent chain complexes were purified. To probe the interactions with the soluble components of the targeting machinery, we used UV-activatable cross-linkers incorporated at specific positions in the nascent chains, as well as conventional sulfhydryl crosslinkers, With both cross-linking approaches, the D1 ribosome nascent chain was photocross-linked to cpSRP54, cpSRP54 was shown to interact only when the D1 nascent chain was still attached to the ribosome, The interaction was strongly dependent on the length of the nascent chain that emerged from the ribosome, as well as the cross-link position. No interactions with soluble SecA or cpSRP43 were found, These results imply a role for cpSRP54 in D1 biogenesis.