Abstract
Members of the Oxa1/YidC/Alb3 protein family are involved in the insertion, folding, and assembly of membrane proteins in mitochondria, bacteria, and chloroplasts. The thylakoid membrane protein Alb3 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/Alb3 insertion machinery. Here, we show that nanodisc-embedded Alb3 interacts with ribosomes, while the homolog Alb4, also located in the thylakoid membrane, shows no ribosome binding. Alb3 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 uL4c is involved in Alb3 binding. Phylogenetic analyses of uL4 demonstrate that this region newly evolved in the green lineage during the transition from aquatic to terrestrial life.
Highlights
The thylakoid membrane of Arabidopsis thaliana harbors the integral membrane proteins Alb3 and Alb4, which belong to the Oxa1/YidC/Alb3 protein family
An extended C-terminal region is absent in E. coli YidC but is characteristic of mitochondrial Oxa1 proteins (Figure 1B)
The cotranslational insertion of membrane proteins requires a tight coupling of the ribosome to the insertase in the target membrane to enable an efficient transfer of the nascent chain into the protein-conducting channel of the insertase
Summary
The thylakoid membrane of Arabidopsis thaliana harbors the integral membrane proteins Alb and Alb, which belong to the Oxa1/YidC/Alb protein family (later referred to as Oxa family). Members of this family are involved in the insertion, folding, and assembly of proteins in mitochondrial, bacterial, and thylakoid membranes (Hennon et al, 2015; McDowell et al, 2021). The LHCPs are first transported across the Toc/Tic translocon of the chloroplast envelope and further directed to the Alb insertase of the thylakoid membrane via the posttranslational chloroplast signal recognition particle (cpSRP). After formation of a soluble LHCP/cpSRP43/cpSRP54 complex, which traverses the stroma, docking to the insertase is mediated by binding of the cpSRP targeting factors to Alb. Direct interactions occur between Alb and cpSRP43 (Bals et al, 2010; Falk et al, 2010; Lewis et al, 2010; Dünschede et al, 2011) as well as between Alb and cpSRP54 in complex with its receptor, cpFtsY (Moore et al, 2003; Chandrasekar and Shan, 2017)
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