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Published January 11, 2008 | Supplemental Material
Journal Article Open

Structure of the Chloroplast Signal Recognition Particle (SRP) Receptor: Domain Arrangement Modulates SRP–Receptor Interaction


The signal recognition particle (SRP) pathway mediates co-translational targeting of nascent proteins to membranes. Chloroplast SRP is unique in that it does not contain the otherwise universally conserved SRP RNA, which accelerates the association between the SRP guanosine-5′-triphosphate (GTP) binding protein and its receptor FtsY in classical SRP pathways. Recently, we showed that the SRP and SRP receptor (SR) GTPases from chloroplast (cpSRP54 and cpFtsY, respectively) can interact with one another 400-fold more efficiently than their bacterial homologues, thus providing an explanation as to why this novel chloroplast SRP pathway bypasses the requirement for the SRP RNA. Here we report the crystal structure of cpFtsY from Arabidopsis thaliana at 2.0 Å resolution. In this chloroplast SR, the N-terminal "N" domain is more tightly packed, and a more extensive interaction surface is formed between the GTPase "G" domain and the N domain than was previously observed in many of its bacterial homologues. As a result, the overall conformation of apo-cpFtsY is closer to that found in the bacterial SRP•FtsY complex than in free bacterial FtsY, especially with regard to the relative orientation of the N and G domains. In contrast, active-site residues in the G domain are mispositioned, explaining the low basal GTP binding and hydrolysis activity of free cpFtsY. This structure emphasizes proper N–G domain arrangement as a key factor in modulating the efficiency of SRP–receptor interaction and helps account, in part, for the faster kinetics at which the chloroplast SR interacts with its binding partner in the absence of an SRP RNA.

Additional Information

© 2007 Elsevier Ltd. Received 24 May 2007. Revised 24 August 2007. Accepted 20 September 2007. Available online 28 September 2007. Edited by J. Doudna. We thank D. C. Rees, P. J. Bjorkman, W. M. Clemons and their laboratory members for extensive assistance throughout this work and for valuable comments on the manuscript; B. Sankaran at the conformation from its E. coli and Taq homologues, and thus further contribute to the efficient association kinetics between cpSRP54 and cpFtsY. Together, the biochemical and crystallographic analyses of cpFtsY emphasize the importance of proper N–G domain arrangement as one of the key factors that modulate the efficiency of interaction between the SRP and SR GTPases and help explain, in part, why the chloroplast SRP pathway can bypass the requirement for an SRP RNA, which is used to accelerate complex formation between the two GTPases in most bacterial SRP pathways.

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