A crystal structure of the human protein kinase Mps1 reveals an ordered conformation of the activation loop

Abstract

Monopolar spindle 1 (Mps1) is a dual-specificity protein kinase, orchestrating faithful chromosome segregation during mitosis. All reported structures of the Mps1 kinase adopt the hallmarks of an inactive conformation, which includes a mostly disordered activation loop. Here, we present a 2.4 Å resolution crystal structure of an "extended" version of the Mps1 kinase domain, which shows an ordered activation loop. However, the other structural characteristics of an active kinase are not present. Our structure shows that the Mps1 activation loop can fit to the ATP binding pocket and interferes with ATP, but less so with inhibitors binding, partly explain the potency of various Mps1 inhibitors.

Keywords: ATP; Mps1; TTK; X-ray crystallography; activation loop; mitotic kinase; spindle assembly checkpoint.

Figure 1

MDS enhances the Mps1 kinase activity. A, Schematic diagram of Mps1 variants. B, FP measurements of turnover of phosphorylation of a Knl1 peptide. Red, green, and blue circles represent the measurements with Mps1^1–808, Mps1^400–808, and Mps1^519–808, respectively. C−E, Cartoon representations of the crystal structure of the Mps1 kinase domain with the ordered activation loop. The activation loop, catalytic loop, phosphate binding loop, and P + 1 loop are highlighted in pink, yellow, orange, and green, respectively. C, Overall structure‐fold of the Mps1 kinase domain. D, ATP binding pocket is depicted as a gray ellipsoid. E, 2mF_o‐DF_c electron density map of the activation loop contoured at 1.0σ

Figure 2

Structure and comparison analysis of the Mps1 kinase domain. The NMS‐P715 bound (5AP113), ADP bound (3HMN6), Cpd5 bound (5O9112), and current structures (6GVJ) are represented as orange, gold, pink, and blue, respectively. Compounds and annotated residues are depicted as sticks. A, Activation loops with and without phosphorylation. The distance between the positions of the Thr 676 backbone in the two loops is represented as a solid line. B−D, The activation loop severely clashes with ADP and moderately with NMS‐P715 and Cpd5. E and F, Characteristics of inactive conformation of the Mps1 kinase. E, The salt bridge between Lys 553 and Glu 571 is disrupted. Instead, they make hydrogen bonds (black dotted lines) with the residues of the activation loop. F, The side chains building hydrophobic R‐spine of the Mps1 kinase are not aligned