title: Chaperone-like properties of tobacco plastid thioredoxins f and m.
abstract: Thioredoxins (Trxs) are ubiquitous disulphide reductases that play important roles in the redox regulation of many cellular processes. However, some redox-independent functions, such as chaperone activity, have also been attributed to Trxs in recent years. The focus of our study is on the putative chaperone function of the well-described plastid Trxs f and m. To that end, the cDNA of both Trxs, designated as NtTrxf and NtTrxm, was isolated from Nicotiana tabacum plants. It was found that bacterially expressed tobacco Trx f and Trx m, in addition to their disulphide reductase activity, possessed chaperone-like properties. In vitro, Trx f and Trx m could both facilitate the reactivation of the cysteine-free form of chemically denatured glucose-6 phosphate dehydrogenase (foldase chaperone activity) and prevent heat-induced malate dehydrogenase aggregation (holdase chaperone activity). Our results led us to infer that the disulphide reductase and foldase chaperone functions prevail when the proteins occur as monomers and the well-conserved non-active cysteine present in Trx f is critical for both functions. By contrast, the holdase chaperone activity of both Trxs depended on their oligomeric status: the proteins were functional only when they were associated with high molecular mass protein complexes. Because the oligomeric status of both Trxs was induced by salt and temperature, our data suggest that plastid Trxs could operate as molecular holdase chaperones upon oxidative stress, acting as a type of small stress protein.
authors.affiliation: Instituto de Agrobiotecnología, Universidad Pública de Navarra-CSIC-Gobierno de Navarra, Campus Arrosadía, E-31006 Pamplona, Spain.
authors.forename: Ruth
authors.initials: R
authors.lastname: Sanz-Barrio
authors.forename: Alicia
authors.initials: A
authors.lastname: Fernández-San Millán
authors.forename: Jon
authors.initials: J
authors.lastname: Carballeda
authors.forename: Patricia
authors.initials: P
authors.lastname: Corral-Martínez
authors.forename: José M
authors.initials: JM
authors.lastname: Seguí-Simarro
authors.forename: Inmaculada
authors.initials: I
authors.lastname: Farran
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D054479:Chloroplast Thioredoxins / Q000502:physiology*; D017931:DNA Primers; D018076:DNA, Complementary; D008958:Models, Molecular; D018832:Molecular Chaperones / Q000502:physiology*; D008969:Molecular Sequence Data; D018087:Plastids; D016133:Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D014026:Nicotiana / Q000235:genetics / Q000502:physiology*
chemical_list: D054479:Chloroplast Thioredoxins; D017931:DNA Primers; D018076:DNA, Complementary; D018832:Molecular Chaperones