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J. Biol. Chem., Vol. 278, Issue 47, 46473-46481, November 21, 2003

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Precursors with Altered Affinity for Hsp70 in Their Transit Peptides Are Efficiently Imported into Chloroplasts^*

Daniela V. Rial, Jorgelina Ottado, and Eduardo A. Ceccarelli¶

From the Molecular Biology Division, Instituto de Biología Molecular y Celular de Rosario, Consejo Nacional de Investigaciones Científicas y Técnicas, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Suipacha 531, S2002LRK Rosario, Argentina

Protein import into chloroplasts is postulated to occur with the involvement of molecular chaperones. We have determined that the transit peptide of ferredoxin-NADP(H) reductase precursor binds preferentially to an Hsp70 from chloroplast stroma. To investigate the role of Hsp70 molecular chaperones in chloroplast protein import, we analyzed the import into pea chloroplasts of preproteins with decreased Hsp70 binding affinity in their transit peptides. Our results indicate that the precursor with the lowest affinity for Hsp70 molecular chaperones in its transit peptide was imported to chloroplasts with similar apparent K_m as the wild type precursor and a 2-fold increase in V_max. Thus, a strong interaction between chloroplast stromal Hsp70 and the transit peptide seems not to be essential for protein import. These results indicate that in chloroplasts the main unfolding force during protein import may be applied by molecular chaperones other than Hsp70s. Although stromal Hsp70s undoubtedly participate in chloroplast biogenesis, the role of these molecular chaperones in chloroplast protein translocation differs from the one proposed in the mechanisms postulated up to date.

Received for publication, June 24, 2003 , and in revised form, September 9, 2003.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AF241251, AF241252, and AF241253.

^* This study was supported in part by grants from the International Foundation for Science (Sweden), the Fundación Antorchas, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), and the Agencia de Promoción Científica y Tecnológica (Argentina). 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.

A Fellow of the CONICET, Argentina.

Staff members of the CONICET, Argentina.

^¶ To whom correspondence should be addressed. Tel.: 54-341-435-1235; Fax: 54-341-439-0465; E-mail: cecca{at}arnet.com.ar.

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