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Published July 2007 | Published + Supplemental Material
Journal Article Open

Efficient Interaction between Two GTPases Allows the Chloroplast SRP Pathway to Bypass the Requirement for an SRP RNA


Cotranslational protein targeting to membranes is regulated by two GTPases in the signal recognition particle (SRP) and the SRP receptor; association between the two GTPases is slow and is accelerated 400-fold by the SRP RNA. Intriguingly, the otherwise universally conserved SRP RNA is missing in a novel chloroplast SRP pathway. We found that even in the absence of an SRP RNA, the chloroplast SRP and receptor GTPases can interact efficiently with one another; the kinetics of interaction between the chloroplast GTPases is 400-fold faster than their bacterial homologues, and matches the rate at which the bacterial SRP and receptor interact with the help of SRP RNA. Biochemical analyses further suggest that the chloroplast SRP receptor is pre-organized in a conformation that allows optimal interaction with its binding partner, so that conformational changes during complex formation are minimized. Our results highlight intriguing differences between the classical and chloroplast SRP and SRP receptor GTPases, and help explain how the chloroplast SRP pathway can mediate efficient targeting of proteins to the thylakoid membrane in the absence of the SRP RNA, which plays an indispensable role in all the other SRP pathways.

Additional Information

Copyright © 2007 by The American Society for Cell Biology. Under the License and Publishing Agreement, authors grant to the general public, effective two months after publication of (i.e.,. the appearance of) the edited manuscript in an online issue of MBoC, the nonexclusive right to copy, distribute, or display the manuscript subject to the terms of the Creative Commons–Noncommercial–Share Alike 3.0 Unported license (http://creativecommons.org/licenses/by-nc-sa/3.0). Submitted January 17, 2007; Revised April 18, 2007; Accepted April 20, 2007. Originally published as MBC in Press, 10.1091/mbc.E07-01-0037 on May 2, 2007. We thank Dr. Ralph Henry (University of Arkansas) for the bacterial expression vectors for cpSRP54 and cpFtsY, Dr. Judith Campbell and the rest of the Shan laboratory for comments on the manuscript, and Dr. Peter Walter for insightful discussions and intellectual support. S.S. was supported by career awards from the Burroughs Wellcome Fund and the Camile and Henry Dreyfus Foundation. P.J.A. is supported by the Bray fellowship. The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

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Published - JARmbc07.pdf

Supplemental Material - JARmbc07suppmat.doc


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