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Bernd Ackermann, Beatrix Dünschede, Björn Pietzenuk, Bo Højen Justesen, Ute Krämer, Eckhard Hofmann, Thomas Günther-Pomorski, Danja Schünemann

• Members of the Oxa1/YidC/Alb\(_3\) protein family are involved in the insertion, folding, and assembly of membrane proteins in mitochondria, bacteria, and chloroplasts. The thylakoid membrane protein Alb\(_3\) mediates the chloroplast signal recognition particle (cpSRP)-dependent posttranslational insertion of nuclear-encoded light harvesting chlorophyll a/b-binding proteins and participates in the biogenesis of plastid-encoded subunits of the photosynthetic complexes. These subunits are cotranslationally inserted into the thylakoid membrane, yet very little is known about the molecular mechanisms underlying docking of the ribosome-nascent chain complexes to the chloroplast SecY/Alb\(_3\) insertion machinery. Here, we show that nanodisc-embedded Alb\(_3\) interacts with ribosomes, while the homolog Alb\(_4\), also located in the thylakoid membrane, shows no ribosome binding. Alb\(_3\) contacts the ribosome with its C-terminal region and at least one additional binding site within its hydrophobic core region. Within the C-terminal region, two conserved motifs (motifs III and IV) are cooperatively required to enable the ribosome contact. Furthermore, our data suggest that the negatively charged C-terminus of the ribosomal subunit uL\(_{4c}\) is involved in Alb\(_3\) binding. Phylogenetic analyses of uL\(_4\) demonstrate that this region newly evolved in the green lineage during the transition from aquatic to terrestrial life.