The chaperone function of ClpB from Thermus thermophilus depends on allosteric interactions of its two ATP-binding sites

Abstract

ClpB belongs to the Hsp100 family and assists de-aggregation of protein aggregates by DnaK chaperone systems. It contains two Walker consensus sequences (or P-Loops) that indicate potential nucleotide binding domains (NBD). Both domains appear to be essential for chaperoning function, since mutation of the conserved lysine residue of the GX 4GKT consensus sequences to glutamine (K204Q and K601Q) abolishes its properties to accelerate renaturation of aggregated firefly luciferase. The underlying biochemical reason for this malfunction appears not to be a dramatically reduced ATPase activity of either P-loop per se but rather changed properties of co-operativity of ATPase activity connected to oligomerization properties to form productive oligomers. This view is corroborated by data that show that structural stability (as judged by CD spectroscopy) or ATPase activity at single turnover conditions (at low ATP concentrations) are not significantly affected by these mutations. In addition nucleotide binding properties of wild-type protein and mutants (as judged by binding studies with fluorescent nucleotide analogues and competitive displacement titrations) do not differ dramatically. However, the general pattern of formation of stable, defined oligomers formed as a function of salt concentration and nucleotides and more importantly, cooperativity of ATPase activity at high ATP concentrations is dramatically changed with the two P-loop mutants described.