The ATPase activity of Hsp104, effects of environmental conditions and mutations

Hsp104 is crucial for stress tolerance in Saccharomyces cerevisiae, and both of its nucleotide-binding domains (NBD1 and NBD2) are required. Here, we characterize the ATPase activity and oligomerization properties of wild-type (WT) Hsp104 and of NBD mutants. In physiological ionic strength buffers (pH 7.5, 37 degreesC) WT Hsp104 exhibits Michaelis-Menten kinetics between 0.5 and 25 mM ATP (Km approximately 5 mM, Vmax approximately 2 nmol min-1 microg-1). ATPase activity is strongly influenced by factors that vary with cell stress (e.g. temperature, pH, and ADP). Mutations in the P-loop of NBD1 (G217V or K218T) severely reduce ATP hydrolysis but have little effect on oligomerization. Analogous mutations in NBD2 (G619V or K620T) have smaller effects on ATPase activity but impair oligomerization. The opposite relationship was reported for another member of the HSP100 protein family, the Escherichia coli ClpA protein, in studies employing lower ionic strength buffers. In such buffers, the Km of WT Hsp104 for ATP hydrolysis decreased 10-fold and its stability under stress conditions increased, but the effects of the NBD mutations on ATPase activity and oligomerization remained opposite to those of ClpA. Either the functions of the two NBDs in ClpA and Hsp104 have been reversed or both contribute to ATP hydrolysis and oligomerization in a complex manner that can be idiosyncratically affected by such mutations.