Role of a Membrane‐activated Structural Switch Mechanism in Chloroplast SRP Receptor Function

Abstract

Prokaryotic and eukaryotic signal recognition particles (SRPs) differ in the ability of the SRP receptor to partition between an aqueous phase and target membranes during protein targeting. Though membrane binding has been shown to be functionally critical for E. coli SRP receptor function, the regulation of the SRP particle receptor partitioning and the conformation of its membrane‐bound state remain unclear. Our study objective was to identify the critical region responsible for membrane binding in the chloroplast SRP (cpSRP) receptor, cpFtsY. Using recombinant cpFtsY, we have identified a small N‐terminal region responsible for stabilizing a membrane interaction critical to the cpFtsY function in cpSRP‐dependent protein targeting. Functional studies of this region reveal that it is both necessary and sufficient for binding the target membrane. Furthermore, NMR and CD structural studies of this region and a similar region in the E. coli SRP receptor reveal a conformational change in secondary structure that takes place upon lipid binding and correlates with a stimulation of GTP hydrolysis by cpFtsY in the presence of liposomes. These studies suggest a conserved mechanism for both membrane binding and the intramolecular communication that regulates cpSRP receptor functions at the membrane. This research is supported by NIH Grant P20 RR15569 from the COBRE Program of the NCRR and DOE Grant DE‐FG02‐01ER15161.