Biochemical and structural characterization of the chloroplast signal recognition particle

The signal recognition particle (SRP) is a ribonucleoprotein complex conserved in all three kingdoms of life¹. The chloroplast signal recognition particle (cpSRP) differs significantly from other SRPs. cpSRP can also act post-translationally – without ribosomes and without SRP RNA. The main substrates for this pathway are the light harvesting chlorophyll a/b binding proteins (LHCPs) that serve as antenna systems in photosynthesis. They comprise three transmembrane helices and an internal targeting signal, the L18 region. LHCPs are nuclear encoded and imported into the chloroplast after synthesis, where they form a soluble transit complex with cpSRP in the stroma. cpSRP is a heterodimer formed by cpSRP54, a homologue of SRP54, and cpSRP43, which is characterized by a unique arrangement of chromodomains and ankyrin repeats. All these domains represent protein-protein interaction modules and thus provide cpSRP43 with a large scaffold for multiple binding partners. The ankyrin repeats are involved in LHCP binding by recognizing a conserved ‘DPLG’ motif in the L18 peptide². Recently, cpSRP43 was shown to be a specific chaperone for LHCP and to be sufficient to keep LHCP in a soluble complex³. cpSRP43 also interacts with the C-terminus of Alb3, thereby recruiting cpSRP to the thylakoid membrane⁴. Interaction studies revealed the second chromodomain as the major binding site for cpSRP54. The binding is mediated by a positively charged 10-amino-acid region, the ‘RRKR’ motif, at the C-terminus of the 54 M domain⁵. Based on biochemical and structural data we show how cpSRP43 adapts the universally conserved SRP system to post-translational targeting and insertion of membrane proteins.