Studies of small molecule interactions with protein phosphatases using biosensor technology

Reversible protein phosphorylation of serine, threonine, and tyrosine residues by protein kinases and phosphatases is important for the regulation of cellular signal transduction and controls many cellular functions. Disturbances in this regulation have been implicated in a growing number of diseases, making kinases and phosphatases useful targets for therapeutic intervention. The suitability of surface plasmon resonance (SPR) technology has been widely demonstrated in many drug discovery applications. A novel and straightforward methodology is presented for analyzing small molecule binding to two serine/threonine phosphatases, PP1 and PP2B (calcineurin), and to the prototypic tyrosine phosphatase, PTP1B. Emphasis was placed on investigating the immobilization conditions of the phosphatases by using reducing conditions, inhibitors and metal ions. A comparison of inhibitor binding, either to phosphatase (PP2B) alone or in complex with the regulatory protein subunit calmodulin, revealed different kinetics. The methodology was also used to test inhibitor specificity toward different phosphatases. Inhibition of regulatory protein PP-inhibitor-2 binding to PP1 by a small molecule inhibitor was demonstrated. This type of information, together with data on compound binding that is independent of enzyme activity and in which affinities are resolved into kinetic rate constants, may be of great significance for the development of highly specific and high-affinity phosphatase inhibitors.