Mps1 Phosphorylates Its N-Terminal Extension to Relieve Autoinhibition and Activate the Spindle Assembly Checkpoint

Abstract

SummaryMonopolar spindle 1 (Mps1) is a conserved apical kinase in the spindle assembly checkpoint (SAC) that ensures accurate segregation of chromosomes during mitosis. Mps1 undergoes extensive auto- and transphosphorylation, but the regulatory and functional consequences of these modifications remain unclear. Recent findings highlight the importance of intermolecular interactions between the N-terminal extension (NTE) of Mps1 and the Hec1 subunit of the NDC80 complex, which control Mps1 localization at kinetochores and activation of the SAC. Whether the NTE regulates other mitotic functions of Mps1 remains unknown. Here, we report that phosphorylation within the NTE contributes to Mps1 activation through relief of catalytic autoinhibition that is mediated by the NTE itself. Moreover, we find that this regulatory NTE function is independent of its role in Mps1 kinetochore recruitment. We demonstrate that the NTE autoinhibitory mechanism impinges most strongly on Mps1-dependent SAC functions and propose that Mps1 activation likely occurs sequentially through dimerization of a “prone-to-autophosphorylate” Mps1 conformer followed by autophosphorylation of the NTE prior to maximal kinase activation segment trans-autophosphorylation. Our observations underline the importance of autoregulated Mps1 activity in generation and maintenance of a robust SAC in human cells.

Kinetochore phosphatases suppress autonomous kinase activity to control the spindle assembly checkpoint

“…Therefore, this creates over a thousand active MELT repeats per kinetochore, which the BUB-PLK1 module can use to rapidly amplifying SAC signalling downstream of MPS1. Furthermore, this may be reinforced by an additional positive feedback loop to MPS1 itself, since PLK1 can phosphorylate the activation loop of MPS1 to stimulate its kinase activity 26,27,37 (MPS1→BUB:PLK1→MPS1, Figure 4B, P2). This may help to explain the recent observation that this 'autoactivation' site remains phosphorylated when MPS1 is inhibited in BUBR1 PP2A cells 38 .…”

Leader of the SAC: molecular mechanisms of Mps1/TTK regulation in mitosis

“…When inactive, both the NTE and MR regions are available, resulting in relatively high affinity of MPS1 for the NDC80 complex, and thus relatively long residence times on kinetochores and high steady-state levels (figure 2E). Activation of MPS1 subsequently causes it to adopt a different conformation [79] that potentially prevents the MR–NUF2 interaction, as recently suggested (figure 2F) [58]. The fully active MPS1 molecule now only interacts with the NDC80 complex via its NTE, lowering overall affinity for the NDC80 complex, and thereby decreasing residence time and levels at mitotic kinetochores (figure 2F).…”

“…The mechanism by which Mps1 dimerizes is unknown, and studies have provided conflicting evidence as to whether the N-terminal region including the TPR domain is involved in this [39,78]. The N-terminal region known as the N-terminal extension (NTE, see below) does affect kinase activation in another way: by directly inhibiting the kinase domain [79]. In any case, dimerization, oligomerization or clustering facilitates activation of Mps1, possibly by releasing NTE-mediated inhibition of the kinase domain and certainly by facilitating trans -autophosphorylation of the activation loop in Mps1.…”

“…This suggests that while the NTE is predominantly responsible for the binding of active MPS1 to kinetochores, both the NTE and MR regions are involved in kinetochore binding of the inactive form (figure 2E). As shown recently, activation of MPS1 leads to extensive conformational changes in the protein [79] which could affect the availability of its kinetochore-binding regions. When inactive, both the NTE and MR regions are available, resulting in relatively high affinity of MPS1 for the NDC80 complex, and thus relatively long residence times on kinetochores and high steady-state levels (figure 2E).…”

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

“…2 Mps1 kinase consists of a series of N-terminal functional modules that ensure its localization to kinetochores, the N-terminal extension (NTE), the tetratricopeptide repeat, and the middle region ( Figure 1A); the NTE motif is reported to contribute to the activation of the kinase domain. 3 The Mps1 degradation signal (MDS) spans across residues 420-507, mediating the regulation of cellular Mps1 levels. 4 The Mps1 kinase domain is at the C-terminal end (519-808) and has been extensively studied, also as a potential drug target for a cancer therapy.…”