The role of chloroplast SRP54 domains and its C-terminal tail region in post- and cotranslational protein transport in vivo

In the chloroplast, the 54 kDa subunit of the signal recognition particle (cpSRP54) is involved in the posttranslational transport of the light-harvesting chlorophyll a/b-binding proteins (LHCPs) and the cotranslational transport of plastid-encoded subunits of the photosynthetic complexes to the thylakoid membrane. It forms a high-affinity complex with plastid-specific cpSRP43 for posttranslational transport, while a ribosome-associated pool coordinates its cotranslational function. CpSRP54 constitutes a conserved multidomain protein, comprising a GTPase (NG) and a methionine-rich (M) domain linked by a flexible region. It is further characterized by a plastid-specific C-terminal tail region containing the cpSRP43-binding motif. To characterize the physiological role of the various regions of cpSRP54 in thylakoid membrane protein transport, we generated Arabidopsis thaliana cpSRP54 knockout ( ffc1-2 ) lines producing truncated cpSRP54 variants or a GTPase point mutation variant. Phenotypic characterization of the complementation lines demonstrated that the C-terminal tail region of cpSRP54 plays an important role specifically in posttranslational LHCP transport. Furthermore, we show that the GTPase activity of cpSRP54 plays an essential role in the transport pathways for both nuclear-as well as plastid-encoded proteins. In addition, our data revealed that plants expressing cpSRP54 without the C-terminal region exhibit a strongly increased accumulation of a photosystem I assembly intermediate.