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J. Biol. Chem., Vol. 279, Issue 41, 43077-43084, October 8, 2004

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Regulation of the GTPase Cycle in Post-translational Signal Recognition Particle-based Protein Targeting Involves cpSRP43^*

Robyn L. Goforth, Eric C. Peterson, Jianguo Yuan, Misty J. Moore, Alicia D. Kight, Matthew B. Lohse, Joshua Sakon, and Ralph L. Henry¶

From the Biological Sciences Department and the Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701

The chloroplast signal recognition particle consists of a conserved 54-kDa GTPase and a novel 43-kDa chromodomain protein (cpSRP43) that together bind light-harvesting chlorophyll a/b-binding protein (LHCP) to form a soluble targeting complex that is subsequently directed to the thylakoid membrane. Homology-based modeling of cpSRP43 indicates the presence of two previously identified chromodomains along with a third N-terminal chromodomain. Chromodomain deletion constructs were used to examine the role of each chromodomain in mediating distinct steps in the LHCP localization mechanism. The C-terminal chromodomain is completely dispensable for LHCP targeting/integration in vitro. The central chromodomain is essential for both targeting complex formation and integration because of its role in binding the M domain of cpSRP54. The N-terminal chromodomain (CD1) is unnecessary for targeting complex formation but is required for integration. This correlates with the ability of CD1 along with the ankyrin repeat region of cpSRP43 to regulate the GTPase cycle of the cpSRP-receptor complex.

Received for publication, February 13, 2004 , and in revised form, July 9, 2004.

^* This work was supported by National Institutes of Health Grant P20 RR15569 from the COBRE Program of the National Center for Research Resources, Department of Energy Grant DE-FG02-01ER15161 (to R. L. H.), and National Science Foundation EPSCoR Grant 9977816. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed: 601 SCEN, Biological Sciences Dept., University of Arkansas, Fayetteville, AR 72701. Tel.: 479-575-2529; Fax: 479-575-4010; E-mail: Ralph.Henry{at}uark.edu.

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