Stochastic detection of Pim protein kinases reveals electrostatically enhanced association of a peptide substrate.

In stochastic sensing, the association and dissociation of analyte molecules is observed as the modulation of an ionic current flowing through a single engineered protein pore, enabling the label-free determination of rate and equilibrium constants with respect to a specific binding site. We engineered sensors based on the staphylococcal α-hemolysin pore to allow the single-molecule detection and characterization of protein kinase-peptide interactions. We enhanced this approach by using site-specific proteolysis to generate pores bearing a single peptide sensor element attached by an N-terminal peptide bond to the trans mouth of the pore. Kinetics and affinities for the Pim protein kinases (Pim-1, Pim-2, and Pim-3) and cAMP-dependent protein kinase were measured and found to be independent of membrane potential and in good agreement with previously reported data. Kinase binding exhibited a distinct current noise behavior that forms a basis for analyte discrimination. Finally, we observed unusually high association rate constants for the interaction of Pim kinases with their consensus substrate Pimtide (~10(7) to 10(8) M(-1) · s(-1)), the result of electrostatic enhancement, and propose a cellular role for this phenomenon.

Expression, purification, crystallization and preliminary crystallographic analysis of human Pim-1 kinase.

Pim kinases, including Pim-1, Pim-2 and Pim-3, belong to a distinctive serine/threonine protein-kinase family. They are involved in cytokine-induced signal transduction and the development of lymphoid malignancies. Their kinase domains are highly homologous to one another, but share low sequence identity to other kinases. Specifically, there are two proline residues in the conserved hinge-region sequence ERPXPX separated by a residue that is non-conserved among Pim kinases. Full-length human Pim-1 kinase (1-313) was cloned and expressed in Escherichia coli as a GST-fusion protein and truncated to Pim-1 (14-313) by thrombin digestion during purification. The Pim-1 (14-313) protein was purified to high homogeneity and monodispersity. This protein preparation yielded small crystals in the initial screening and large crystals after optimization. The large crystals of apo Pim-1 enzyme diffracted to 2.1 A resolution and belong to space group P6(5), with unit-cell parameters a = b = 95.9, c = 80.0 A, beta = 120 degrees and one molecule per asymmetric unit.