Transient Protein-Protein Interactions and a Random-ordered Kinetic Mechanism for the Phosphorylation of a Transcription Factor by Extracellular-regulated Protein Kinase 2

No thorough mechanistic study of extracellular signal-regulated protein kinase 2 (ERK2) has appeared in the literature. A recombinant protein termed EtsΔ138, which comprises of residues 1–138 of the transcription factor Ets-1 is an excellent substrate of ERK2 (Waas W. F., and Dalby, K. N. (2001)Protein Exp. Purif. 23, 191–197). The kinetic mechanism of ERK2 was examined, with excess magnesium, by initial velocity measurements, both in the absence and presence of products at 27 °C, pH 7.5, and ionic strength 0.1 m (KCl). The velocity data are consistent with a steady-state random-ordered ternary complex mechanism, where both substrates have unhindered access to binding sites on the enzyme. The mechanism and magnitude of product inhibition by monophosphorylated EtsΔ138 is consistent with, but does not prove, the notion that ERK2 forms a discrete interaction with EtsΔ138 in the absence of active site interactions, and that this “docking complex” facilitates intramolecular phosphorylation of the substrate. The approximation of the steady-state data to a rapid equilibrium model strongly suggests that the formation of ERK2·Ets138 complexes are transient in nature with dissociation constants of greater magnitude than the catalytic constant, of kcat = 17 s−1.