title: Thermodynamic characterization of nucleoplasmin unfolding: interplay between function and stability.
abstract: The unfolding equilibrium of recombinant (rNP) and natural variants of nucleoplasmin (NP) from Xenopus laevis has been analyzed using biochemical and spectroscopic techniques. In the presence of denaturing concentrations of guanidinium salts (GuHCl and GuSCN), both domains, core and tail, of the rNP pentamer unfold as proven using single-carrying tryptophan mutants, whereas urea is remarkably unable to fully unfold rNP. Chemical unfolding is reversible and can be described well as a two-state transition in which the folded pentamer is directly converted to unfolded monomers, with no evidence of (partially) folded monomers. Therefore, rNP dissociates and fully unfolds simultaneously (N 5 <--> 5U). Activation of the protein by hyperphosphorylation is accompanied by a destabilization of the protein oligomer. A comparison of natural NP forms isolated from eggs and oocytes of X. laevis and recombinant NP reveals that natural variants can be fully unfolded by urea and exhibit D 50 (denaturant concentration at the transition midpoint) values lower than that of the nonphosphorylated protein. Progressive phosphorylation of NP correlates with a gradual loss of stability of 6 kcal/mol (oNP) and 10 kcal/mol (eNP), as compared with the nonphosphorylated protein pentamer. These results suggest that the remarkable stability of the recombinant protein is required to cope with the destabilization brought about by its phosphorylation-induced activation.
authors.affiliation: Unidad de Biofísica (CSIC-UPV/EHU) and Departamento de Bioquímica y Biología Molecular, Universidad del País Vasco, Apdo 644, 48080 Bilbao, Spain.
authors.forename: Guillermo
authors.initials: G
authors.lastname: Franco
authors.forename: Sonia
authors.initials: S
authors.lastname: Bañuelos
authors.forename: Jorge
authors.initials: J
authors.lastname: Falces
authors.forename: Arturo
authors.initials: A
authors.lastname: Muga
authors.forename: María A
authors.initials: MA
authors.lastname: Urbaneja
mesh_terms: D000818:Animals; D002850:Chromatography, Gel; D002942:Circular Dichroism; D005260:Female; D019791:Guanidine / Q000494:pharmacology; D007700:Kinetics; D009154:Mutation; D009687:Nuclear Proteins / Q000737:chemistry* / Q000235:genetics; D056489:Nucleoplasmins; D010750:Phosphoproteins / Q000737:chemistry* / Q000235:genetics; D010766:Phosphorylation; D011487:Protein Conformation / Q000187:drug effects; D011489:Protein Denaturation / Q000187:drug effects; D017510:Protein Folding; D011994:Recombinant Proteins / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D014461:Ultracentrifugation; D014508:Urea / Q000494:pharmacology; D029867:Xenopus Proteins / Q000737:chemistry* / Q000235:genetics; D014982:Xenopus laevis
chemical_list: D009687:Nuclear Proteins; D056489:Nucleoplasmins; D010750:Phosphoproteins; D011994:Recombinant Proteins; D029867:Xenopus Proteins; D014508:Urea; D019791:Guanidine