Xuelian Luo

The Mad2 Spindle Checkpoint Protein Undergoes Similar Major Conformational Changes Upon Binding to Either Mad1 or Cdc20

Mad2 participates in spindle checkpoint inhibition of APC(Cdc20). We show that RNAi-mediated suppression of Mad1 function in mammalian cells causes loss of Mad2 kinetochore localization and impairment of the spindle checkpoint. Mad1 and Cdc20 contain Mad2 binding motifs that share a common consensus. We have identified a class of Mad2 binding peptides with a similar consensus. Binding of one of these ligands, MBP1, triggers an extensive rearrangement of the tertiary structure of Mad2. Mad2 also undergoes a similar striking structural change upon binding to a Mad1 or Cdc20 binding motif peptide. Our data suggest that, upon checkpoint activation, Mad1 recruits Mad2 to unattached kinetochores and may promote binding of Mad2 to Cdc20.

The Mad2 spindle checkpoint protein has two distinct natively folded states.

The spindle checkpoint delays chromosome segregation in response to misaligned sister chromatids during mitosis, thus ensuring the fidelity of chromosome inheritance. Through binding to Cdc20, the Mad2 spindle checkpoint protein inhibits the target of this checkpoint, the ubiquitin protein ligase APC/CCdc20. We now show that without cofactor binding or covalent modification Mad2 adopts two distinct folded conformations at equilibrium (termed N1-Mad2 and N2-Mad2). The structure of N2-Mad2 has been determined by NMR spectroscopy. N2-Mad2 is much more potent in APC/C inhibition. Overexpression of a Mad2 mutant that specifically sequesters N2-Mad2 partially blocks checkpoint signaling in living cells. The two Mad2 conformers interconvert slowly in vitro, but interconversion is accelerated by a fragment of Mad1, an upstream regulator of Mad2. Our results suggest that the unusual two-state behavior of Mad2 is critical for spindle checkpoint signaling.

Conformation‐specific binding of p31comet antagonizes the function of Mad2 in the spindle checkpoint

The spindle checkpoint ensures accurate chromosome segregation by delaying anaphase in response to misaligned sister chromatids during mitosis. Upon checkpoint activation, Mad2 binds directly to Cdc20 and inhibits the anaphase‐promoting complex or cyclosome (APC/C). Cdc20 binding triggers a dramatic conformational change of Mad2. Consistent with an earlier report, we show herein that depletion of p31comet (formerly known as Cmt2) by RNA interference in HeLa cells causes a delay in mitotic exit following the removal of nocodazole. Purified recombinant p31comet protein antagonizes the ability of Mad2 to inhibit APC/CCdc20 in vitro and in Xenopus egg extracts. Interestingly, p31comet binds selectively to the Cdc20‐bound conformation of Mad2. Binding of p31comet to Mad2 does not prevent the interaction between Mad2 and Cdc20 in vitro. During checkpoint inactivation in HeLa cells, p31comet forms a transient complex with APC/CCdc20‐bound Mad2. Purified p31comet enhances the activity of APC/C isolated from nocodazole‐arrested HeLa cells without disrupting the Mad2–Cdc20 interaction. Therefore, our results suggest that p31comet counteracts the function of Mad2 and is required for the silencing of the spindle checkpoint.

Structure of the Mad2 spindle assembly checkpoint protein and its interaction with Cdc20.

The checkpoint protein Mad2 inhibits the activity of the anaphase promoting complex by sequestering Cdc20 until all chromosomes are aligned at the metaphase plate. We report the solution structure of human Mad2 and its interaction with Cdc20. Mad2 possesses a novel three-layered α/β fold with three α-helices packed between two β-sheets. Using deletion mutants we identified the minimal Mad2-binding region of human Cdc20 as a 40-residue segment immediately N-terminal to the WD40 repeats. Mutagenesis and NMR titration experiments show that a C-terminal flexible region of Mad2 is required for binding to Cdc20. Mad2 and Cdc20 form a tight 1:1 heterodimeric complex in which the C-terminal segment of Mad2 becomes folded. These results provide the first structural insight into mechanisms of the spindle assembly checkpoint.

Protein metamorphosis: the two-state behavior of Mad2.

A given protein generally has only one native tertiary fold, which is the conformation with the lowest Gibbs free energy. Mad2, a protein involved in the spindle checkpoint, however, has two natively folded states with similar Gibbs free energies. Through binding to its target Cdc20, Mad2 inhibits the multisubunit ubiquitin ligase, the anaphase-promoting complex or cyclosome (APC/C), and delays the onset of anaphase until all sister chromatids achieve bipolar attachment to the mitotic spindle. Without ligand binding or covalent modifications, Mad2 adopts two topologically and functionally distinct native folds in equilibrium under physiological conditions. The transition between the two Mad2 states is regulated by multiple mechanisms and is central to the activation and inactivation of the spindle checkpoint. This review summarizes recent structural and biochemical studies on the two-state behavior of Mad2 and discusses the generality and implications of structural malleability of proteins.

Structural and functional analysis of the YAP-binding domain of human TEAD2

The Hippo pathway controls organ size and suppresses tumorigenesis in metazoans by blocking cell proliferation and promoting apoptosis. The TEAD1-4 proteins (which contain a DNA-binding domain but lack an activation domain) interact with YAP (which lacks a DNA-binding domain but contains an activation domain) to form functional heterodimeric transcription factors that activate proliferative and prosurvival gene expression programs. The Hippo pathway inhibits the YAP-TEAD hybrid transcription factors by phosphorylating and promoting cytoplasmic retention of YAP. Here we report the crystal structure of the YAP-binding domain (YBD) of human TEAD2. TEAD2 YBD adopts an immunoglobulin-like β-sandwich fold with two extra helix-turn-helix inserts. NMR studies reveal that the TEAD-binding domain of YAP is natively unfolded and that TEAD binding causes localized conformational changes in YAP. In vitro binding and in vivo functional assays define an extensive conserved surface of TEAD2 YBD as the YAP-binding site. Therefore, our studies suggest that a short segment of YAP adopts an extended conformation and forms extensive contacts with a rigid surface of TEAD. Targeting a surface-exposed pocket of TEAD might be an effective strategy to disrupt the YAP-TEAD interaction and to reduce the oncogenic potential of YAP.

Synergistic blockade of mitotic exit by two chemical inhibitors of the APC/C

Protein machines are multi-subunit protein complexes that orchestrate highly regulated biochemical tasks. An example is the anaphase-promoting complex/cyclosome (APC/C), a 13-subunit ubiquitin ligase that initiates the metaphase–anaphase transition and mitotic exit by targeting proteins such as securin and cyclin B1 for ubiquitin-dependent destruction by the proteasome. Because blocking mitotic exit is an effective approach for inducing tumour cell death, the APC/C represents a potential novel target for cancer therapy. APC/C activation in mitosis requires binding of Cdc20 (ref. 5), which forms a co-receptor with the APC/C to recognize substrates containing a destruction box (D-box). Here we demonstrate that we can synergistically inhibit APC/C-dependent proteolysis and mitotic exit by simultaneously disrupting two protein–protein interactions within the APC/C–Cdc20–substrate ternary complex. We identify a small molecule, called apcin (APC inhibitor), which binds to Cdc20 and competitively inhibits the ubiquitylation of D-box-containing substrates. Analysis of the crystal structure of the apcin–Cdc20 complex suggests that apcin occupies the D-box-binding pocket on the side face of the WD40-domain. The ability of apcin to block mitotic exit is synergistically amplified by co-addition of tosyl-l-arginine methyl ester, a small molecule that blocks the APC/C–Cdc20 interaction. This work suggests that simultaneous disruption of multiple, weak protein–protein interactions is an effective approach for inactivating a protein machine.

Insights into Mad2 Regulation in the Spindle Checkpoint Revealed by the Crystal Structure of the Symmetric Mad2 Dimer

In response to misaligned sister chromatids during mitosis, the spindle checkpoint protein Mad2 inhibits the anaphase-promoting complex or cyclosome (APC/C) through binding to its mitotic activator Cdc20, thus delaying anaphase onset. Mad1, an upstream regulator of Mad2, forms a tight core complex with Mad2 and facilitates Mad2 binding to Cdc20. In the absence of its binding proteins, free Mad2 has two natively folded conformers, termed N1-Mad2/open-Mad2 (O-Mad2) and N2-Mad2/closed Mad2 (C-Mad2), with C-Mad2 being more active in APC/CCdc20 inhibition. Here, we show that whereas O-Mad2 is monomeric, C-Mad2 forms either symmetric C-Mad2–C-Mad2 (C–C) or asymmetric O-Mad2–C-Mad2 (O–C) dimers. We also report the crystal structure of the symmetric C–C Mad2 dimer, revealing the basis for the ability of unliganded C-Mad2, but not O-Mad2 or liganded C-Mad2, to form symmetric dimers. A Mad2 mutant that predominantly forms the C–C dimer is functional in vitro and in living cells. Finally, the Mad1–Mad2 core complex facilitates the conversion of O-Mad2 to C-Mad2 in vitro. Collectively, our results establish the existence of a symmetric Mad2 dimer and provide insights into Mad1-assisted conformational activation of Mad2 in the spindle checkpoint.

Structure of human Mad1 C-terminal domain reveals its involvement in kinetochore targeting.

The spindle checkpoint prevents aneuploidy by delaying anaphase onset until all sister chromatids achieve proper microtubule attachment. The kinetochore-bound checkpoint protein complex Mad1-Mad2 promotes the conformational activation of Mad2 and serves as a catalytic engine of checkpoint signaling. How Mad1 is targeted to kinetochores is not understood. Here, we report the crystal structure of the conserved C-terminal domain (CTD) of human Mad1. Mad1 CTD forms a homodimer and, unexpectedly, has a fold similar to those of the kinetochore-binding domains of Spc25 and Csm1. Nonoverlapping Mad1 fragments retain detectable kinetochore targeting. Deletion of the CTD diminishes, does not abolish, Mad1 kinetochore localization. Mutagenesis studies further map the functional interface of Mad1 CTD in kinetochore targeting and implicate Bub1 as its receptor. Our results indicate that CTD is a part of an extensive kinetochore-binding interface of Mad1, and rationalize graded kinetochore targeting of Mad1 during checkpoint signaling.

Structural analysis of human Cdc20 supports multisite degron recognition by APC/C.

The anaphase-promoting complex/cyclosome (APC/C) promotes anaphase onset and mitotic exit through ubiquitinating securin and cyclin B1. The mitotic APC/C activator, the cell division cycle 20 (Cdc20) protein, directly interacts with APC/C degrons––the destruction (D) and KEN boxes. APC/CCdc20 is the target of the spindle checkpoint. Checkpoint inhibition of APC/CCdc20 requires the binding of a BubR1 KEN box to Cdc20. How APC/C recognizes substrates is not understood. We report the crystal structures of human Cdc20 alone or bound to a BubR1 KEN box. Cdc20 has a disordered N-terminal region and a C-terminal WD40 β propeller with a preformed KEN-box-binding site at its top face. We identify a second conserved surface at the side of the Cdc20 β propeller as a D-box-binding site. The D box of securin, but not its KEN box, is critical for securin ubiquitination by APC/CCdc20. Although both motifs contribute to securin ubiquitination by APC/CCdh1, securin mutants lacking either motif are efficiently ubiquitinated. Furthermore, D-box peptides diminish the ubiquitination of KEN-box substrates by APC/CCdh1, suggesting possible competition between the two motifs. Our results indicate the lack of strong positive cooperativity between the two degrons of securin. We propose that low-cooperativity, multisite target recognition enables APC/C to robustly ubiquitinate diverse substrates and helps to drive cell cycle oscillations.