The cpSRP54‐cpFtsY Interaction in the Chloroplast SRP Pathway

Abstract

The universally conserved signal recognition particle (SRP) and SRP receptor (SR) mediate the cotranslational targeting of proteins to cellular membranes. In contrast, a recently discovered unique chloroplast SRP in green plants is primarily dedicated to the post‐translational targeting of light harvesting chlorophyll a/b binding (LHC) proteins to the thylakoid membranes. The pathway involves cpSRP54 and its receptor cpFtsY, two GTPases that are similar to the cytosolic SRP54 and SR GTPases. Intriguingly, the otherwise universally conserved SRP RNA which accelerates complex formation between the two GTPases has not been found in the chloroplast pathway. Instead, a new 43‐kDa protein, cpSRP43, forms the chloroplast SRP together with cpSRP54. Kinetic analyses have shown that cpSRP43 does not significantly affect the rate of the cpSRP54·cpFtsY complex formation or the GTP hydrolysis rate of the complex. Instead, cpSRP43 facilitates the substrate recognition through interactions with the cargo proteins, LHCPs. This project seeks to understand how cpSRP54 and cpFtsY are able to associate more efficiently than their bacterial homologues in the absence of the SRP RNA by studying their complex structure using crystallography and lysine footprinting mass spectrometry. To date, 2‐dimensional crystal plates have been obtained, and progress continues toward the determination of a cpSRP54·cpFtsY structure.