Activated ERK2 Is a Monomer in Vitro with or without Divalent Cations and When Complexed to the Cytoplasmic Scaffold PEA-15

The extracellular signal-regulated protein kinase, ERK2, fully activated by phosphorylation and without a His(6) tag, shows little tendency to dimerize with or without either calcium or magnesium ions when analyzed by light scattering or analytical ultracentrifugation. Light scattering shows that similar to 90% of ERK2 is monomeric. Sedimentation equilibrium data (obtained at 4.8-11.2,mu M ERK2) with or without magnesium (10 mM) are well described by an ideal one-component model with a fitted molar mass of 40180 +/- 240 Da (without Mg2+ ions) or 41290 +/- 330 Da (with Mg2+ ions). These values, close to the sequence-derived mass of 41711 Da, indicate that no significant dimerization of ERK2 occurs in solution. Analysis of sedimentation velocity data for a 15 mu M solution of ERK2 with an enhanced van Holde-Weischet method determined the sedimentation coefficient (s) to be similar to 3.22 S for activated ERK2 with or without 10 mM MgCl2. The frictional coefficient ratio (f/f(0)) of 1.28 calculated from the sedimentation velocity and equilibrium data is close to that expected for an similar to 42 kDa globular protein. The translational diffusion coefficient of similar to 8.3 x 10(-7) cm(2) s(-1) calculated from the experimentally determined molar mass and sedimentation coefficient agrees with the value determined by dynamic light scattering in the absence and presence of calcium or magnesium ions and a value determined by NMR spectrometry. ERK2 has been proposed to homodimerize and bind only to cytoplasmic but not nuclear proteins [Casar, B., et al. (2008) Mol. Cell 31, 708-721]. Our light scattering data show, however, that ERK2 forms a strong 1:1 complex of similar to 57 kDa with the cytoplasmic scaffold protein PEA-15. Thus, ERK2 binds PEA-15 as a monomer. Our data provide strong evidence that ERK2 is monomeric under physiological conditions. Analysis of the same ERK2 construct with the nonphysiological His(6) tag shows substantial dimerization under the same ionic conditions.