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MBC in Press, published online ahead of print May 2, 2007
Mol. Biol. Cell 10.1091/mbc.E07-01-0037

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Submitted on January 17, 2007
Revised on April 18, 2007
Accepted on April 20, 2007

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

Peera Jaru-Ampornpan, Sowmya Chandrasekar, and Shu-ou Shan

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125

Monitoring Editor: Reid Gilmore

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.

Address correspondence to: Shu-ou Shan (sshan{at}caltech.edu)




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K. F. Stengel, I. Holdermann, P. Cain, C. Robinson, K. Wild, and I. Sinning
Structural Basis for Specific Substrate Recognition by the Chloroplast Signal Recognition Particle Protein cpSRP43
Science, July 11, 2008; 321(5886): 253 - 256.
[Abstract] [Full Text] [PDF]


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