Co-evolution of two GTPases enables efficient protein targeting in an RNA-less chloroplast Signal Recognition Particle pathway

Creators: Chandrasekar, Sowmya; Sweredoski, Michael J.; Sohn, Chang Ho; Hess, Sonja; Shan, Shu-ou

Abstract

The signal recognition particle (SRP) is an essential ribonucleoprotein particle that mediates the co-translational targeting of newly synthesized proteins to cellular membranes. The SRP RNA is a universally conserved component of SRP that mediates key interactions between two GTPases in SRP and its receptor, thus enabling rapid delivery of cargo to the target membrane. Notably, this essential RNA is bypassed in the chloroplast (cp) SRP of green plants. Previously, we showed that the cpSRP and cpSRP receptor GTPases (cpSRP54 and cpFtsY, respectively) interact efficiently by themselves without the SRP RNA. Here, we explore the molecular mechanism by which this is accomplished. Fluorescence analyses showed that, in the absence of SRP RNA, the M-domain of cpSRP54 both accelerates and stabilizes complex assembly between cpSRP54 and cpFtsY. Cross-linking coupled with mass spectrometry and mutational analyses identified a new interaction between complementarily charged residues on the cpFtsY G-domain and the vicinity of the cpSRP54 M-domain. These residues are specifically conserved in plastids, and their evolution coincides with the loss of SRP RNA in green plants. These results provide an example of how proteins replace the functions of RNA during evolution.

Additional Information

© 2017 The American Society for Biochemistry and Molecular Biology, Inc. Received August 9, 2016; Revision received November 17, 2016; First Published on November 28, 2016. This work was supported by National Institutes of Health Grant 1R01GM114390 and fellowships from the Gordon and Betty Moore Foundation and American Federation for Aging Research (to S. S.). The authors declare that they have no conflicts of interest with the contents of this article. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. We thank Jesse Beauchamp for helpful discussions on the cross-linking and mass spectrometry experiments; members of the Proteome Exploration Laboratory for help with mass spectrometry experiments and analyses; the Guttman laboratory for help with phylogenic analysis, and members of the Shan laboratory for comments on the manuscript. The Proteome Exploration Laboratory was supported by the Beckman Institute and the Gordon and Betty Moore Foundation through Grant GBMF775. Author Contributions: S. S. conceived and coordinated the study and wrote the paper. S. C. designed, performed, and analyzed the experiments shown in Figs. 1–5, carried out the analyses in Fig. 6 and Table 1, and wrote the paper. S. C., C. H. S., S. H., and M. J. S. designed, performed, and analyzed the experiments shown in Fig. 2 and Table 1. All authors reviewed the results and approved the final version of the manuscript.

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