Thermodynamics and Kinetics of Hsp70 Association with A + U-rich mRNA-destabilizing Sequences*

Rapid mRNA degradation directed by A + U-rich elements (AREs) is mediated by the interaction of specific RNA-binding proteins to these sequences. The protein chaperone Hsp70 has been identified in a cellular complex containing the ARE-binding protein AUF1 and has also been detected in direct contact with A + U-rich RNA substrates, indicating that Hsp70 may be involved in the regulation of ARE-directed mRNA turnover. By using gel mobility shift and fluorescence anisotropy assays, we have determined that Hsp70 directly and specifically associates with U-rich RNA substrates in solution. With the ARE from tumor necrosis factor α (TNFα) mRNA, Hsp70 forms a dynamic complex consistent with a 1:1 association of protein:RNA but demonstrates cooperative binding behavior on polyuridylate substrates. Unlike AUF1, the RNA binding activity of Hsp70 is not regulated by ion-dependent folding of the TNFα ARE, suggesting that AUF1 and Hsp70 recognize distinct binding determinants on this RNA substrate. Binding of Hsp70 to the TNFα ARE is driven entirely by enthalpy at physiological temperatures, indicating that burial of hydrophobic surfaces is likely the principal mechanism stabilizing the Hsp70·RNA complex. Potential roles for the interaction of Hsp70 with ARE-containing mRNAs in the regulation of mRNA turnover and/or translational efficiency are discussed.