Elyse S. Fischer

Bioluminescent systems are widely used as molecular reporters in life sciences due to their low background, broad dynamic range, and the key advantage of not requiring external excitation. However, natural luciferases often lack the brightness, stability, and long-lasting light emission needed to support a wide range of high-sensitivity applications. Here, we describe our next-generation luciferase solution: LuxSit Pro and its substrate, Luxterazine (LTZ). LuxSit Pro is a de novo luciferase optimized for steady brightness and structural stability through an iterative combination of computational and experimental methods. Composed of only 117 amino acids, LuxSit Pro is currently the smallest highest-performing known luciferase, exhibiting exceptional brightness and biochemical stability with robust recombinant expression in both mammalian and bacterial systems. Notably, LuxSit Pro was deliberately designed devoid of lysine and cysteine residues which enormously expands its potential applications. This property renders the reporter largely resistant to post-translational modifications such as ubiquitination and also enables site-directed chemical modifications, as lysines or cysteines can be reintroduced at specific sites to serve as chemical handles for augmenting or adapting the enzyme’s function. The novel synthetic substrate, Luxterazine (LTZ), is a compound that enables LuxSit Pro to deliver bright, steady, blue light emission (∼490 nm) and offers improved solubility compared to existing Coelenterazine analogs. Finally, we also developed LuxSit sPro, a highly sensitive two-component complementation system for the quantitative detection of molecular interactions. The broad potential applications of LuxSit Pro and sPro, in combination with Luxterazine, are poised to drive the development of next-generation reporters and biosensor systems for the research and in vitro diagnostics fields.

During mitosis, unattached kinetochores trigger the spindle assembly checkpoint by promoting assembly of the mitotic checkpoint complex, a heterotetramer comprising Mad2, Cdc20, BubR1 and Bub3. Critical to this process is the kinetochore-mediated catalysis of an intrinsically slow conformational conversion of Mad2 from an open (O-Mad2) inactive state to a closed (C-Mad2) active state bound to Cdc20. These Mad2 conformational changes involve substantial remodelling of the N-terminal β1 strand and C-terminal β7/β8 hairpin. In vitro , the Mad2- interaction motif (MIM) of Cdc20 (Cdc20 ^MIM ) triggers rapid conversion of O- to C-Mad2, effectively removing the kinetic barrier for MCC assembly. How Cdc20 ^MIM directly induces Mad2 conversion remains unclear. In this study we demonstrate that the Cdc20 ^MIM -binding site is inaccessible in O-Mad2. Time-resolved NMR and molecular dynamics simulations show how Mad2 conversion involves sequential conformational changes of flexible structural elements in O-Mad2, orchestrated by Cdc20 ^MIM . Conversion is initiated by the β7/β8 hairpin of O-Mad2 transiently unfolding to expose a nascent Cdc20 ^MIM -binding site. Engagement of Cdc20 ^MIM to this site promotes release of the β1 strand. We propose that initial conformational changes of the β7/β8 hairpin allows binding of Cdc20 ^MIM to a transient intermediate state of Mad2, thereby lowering the kinetic barrier to Mad2 conversion.

The spindle assembly checkpoint (SAC) is a cellular surveillance mechanism that functions to ensure accurate chromosome segregation during mitosis. Macromolecular complexes known as kinetochores, act as the interface of sister chromatid attachment to spindle microtubules. In response to unattached kinetochores, the SAC activates its effector, the mitotic checkpoint complex (MCC), which delays mitotic exit until all sister chromatid pairs have achieved successful attachment to the bipolar mitotic spindle. Formation of the MCC (composed of Mad2, BubR1, Bub3 and Cdc20) is regulated by an Mps1 kinase‐dependent phosphorylation signaling cascade which assembles and repositions components of the MCC onto a catalytic scaffold. This scaffold functions to catalyze the conversion of the HORMA‐domain protein Mad2 from an “inactive” open‐state (O‐Mad2) into an “active” closed‐Mad2 (C‐Mad2), and simultaneous Cdc20 binding. Here, our current understanding of the molecular mechanisms underlying the kinetic barrier to C‐Mad2:Cdc20 formation will be reviewed. Recent progress in elucidating the precise molecular choreography orchestrated by the catalytic scaffold to rapidly assemble the MCC will be examined, and unresolved questions will be highlighted. Ultimately, understanding how the SAC rapidly activates the checkpoint not only provides insights into how cells maintain genomic integrity during mitosis, but also provides a paradigm for how cells can utilize molecular switches, including other HORMA domain‐containing proteins, to make rapid changes to a cell's physiological state.

In response to improper kinetochore-microtubule attachments in mitosis, the spindle assembly checkpoint (SAC) assembles the mitotic checkpoint complex (MCC) to inhibit the anaphase-promoting complex/cyclosome, thereby delaying entry into anaphase. The MCC comprises Mad2:Cdc20:BubR1:Bub3. Its assembly is catalysed by unattached kinetochores on a Mad1:Mad2 platform. Mad1-bound closed-Mad2 (C-Mad2) recruits open-Mad2 (O-Mad2) through self-dimerization. This interaction, combined with Mps1 kinase-mediated phosphorylation of Bub1 and Mad1, accelerates MCC assembly, in a process that requires O-Mad2 to C-Mad2 conversion and concomitant binding of Cdc20. How Mad1 phosphorylation catalyses MCC assembly is poorly understood. Here, we characterized Mps1 phosphorylation of Mad1 and obtained structural insights into a phosphorylation-specific Mad1:Cdc20 interaction. This interaction, together with the Mps1-phosphorylation dependent association of Bub1 and Mad1, generates a tripartite assembly of Bub1 and Cdc20 onto the C-terminal domain of Mad1 (Mad1^CTD). We additionally identify flexibility of Mad1:Mad2 that suggests how the Cdc20:Mad1^CTD interaction brings the Mad2-interacting motif (MIM) of Cdc20 near O-Mad2. Thus, Mps1-dependent formation of the MCC-assembly scaffold functions to position and orient Cdc20 MIM near O-Mad2, thereby catalysing formation of C-Mad2:Cdc20.

In response to improper kinetochore-microtubule attachments in mitosis, the spindle assembly checkpoint (SAC) assembles the mitotic checkpoint complex (MCC) to inhibit the anaphase- promoting complex/cyclosome, thereby delaying entry into anaphase. The MCC is a complex of Mad2, Cdc20 and BubR1:Bub3. Its assembly is catalysed by unattached kinetochores on a catalytic Mad1:Mad2 platform. Mad1-bound closed-Mad2 (C-Mad2) recruits open-Mad2 (O- Mad2) through self-dimerization. This interaction, combined with Mps1 kinase-mediated phosphorylation of Bub1 and Mad1, accelerates MCC assembly, in a process that requires conversion of O-Mad2 to C-Mad2 and concomitant binding of Cdc20. How Mad1 phosphorylation catalyses MCC assembly is poorly understood. In this study we characterized Mps1 phosphorylation of Mad1 and obtained structural insights into a phosphorylation-specific Mad1:Cdc20 interaction. This interaction, together with the Mps1-phosphorylation dependent association of Bub1 and Mad1, generates a tripartite assembly of Bub1 and Cdc20 onto the C- terminal domain of Mad1 (Mad1 ^CTD ). We additionally identified a folded state of the Mad1:Mad2 complex that suggests how the Cdc20:Mad1 ^CTD interaction brings the Mad2- interacting motif (MIM) of Cdc20 (Cdc20 ^MIM ) into close proximity with O-Mad2. We also show that Cdc20 ^MIM is sufficient to catalyse Mad2 conversion and its entrapment by the Mad2 safety- belt. Thus, Mps1-dependent formation of the MCC-assembly scaffold functions to position and orient Cdc20 ^MIM near O-Mad2, thereby catalysing formation of C-Mad2:Cdc20.

During metaphase, in response to improper kinetochore-microtubule attachments, the spindle assembly checkpoint (SAC) activates the mitotic checkpoint complex (MCC), an inhibitor of the anaphase-promoting complex/cyclosome (APC/C). This process is orchestrated by the kinase Mps1, which initiates the assembly of the MCC onto kinetochores through a sequential phosphorylation-dependent signalling cascade. The Mad1-Mad2 complex, which is required to catalyse MCC formation, is targeted to kinetochores through a direct interaction with the phosphorylated conserved domain 1 (CD1) of Bub1. Here, we present the crystal structure of the C-terminal domain of Mad1 (Mad1 ^CTD ) bound to two phosphorylated Bub1 ^CD1 peptides at 1.75 Å resolution. This interaction is mediated by phosphorylated Bub1 Thr461, which not only directly interacts with Arg617 of the Mad1 RLK (Arg-Leu-Lys) motif, but also directly acts as an N-terminal cap to the CD1 α-helix dipole. Surprisingly, only one Bub1 ^CD1 peptide binds to the Mad1 homodimer in solution. We suggest that this stoichiometry is due to inherent asymmetry in the coiled-coil of Mad1 ^CTD and has implications for how the Mad1-Bub1 complex at kinetochores promotes efficient MCC assembly.