The phytohormone abscisic acid (ABA) mediates the adaptation of plants to environmental stresses such as drought and regulates developmental signals such as seed maturation. Within plants, the PYR/PYL/RCAR family of START proteins receives ABA to inhibit the phosphatase activity of the group-A protein phosphatases 2C (PP2Cs), which are major negative regulators in ABA signalling. Here we present the crystal structures of the ABA receptor PYL1 bound with (+)-ABA, and the complex formed by the further binding of (+)-ABA-bound PYL1 with the PP2C protein ABI1. PYL1 binds (+)-ABA using the START-protein-specific ligand-binding site, thereby forming a hydrophobic pocket on the surface of the closed lid. (+)-ABA-bound PYL1 tightly interacts with a PP2C domain of ABI1 by using the hydrophobic pocket to cover the active site of ABI1 like a plug. Our results reveal the structural basis of the mechanism of (+)-ABA-dependent inhibition of ABI1 by PYL1 in ABA signalling.

Strigolactones (SLs) are phytohormones that inhibit shoot branching and function in the rhizospheric communication with symbiotic fungi and parasitic weeds. An a/b-hydrolase protein, DWARF14 (D14), has been recognized to be an essential component of plant SL signalling, although its precise function remains unknown. Here we present the SL-dependent interaction of D14 with a gibberellin signalling repressor SLR1 and a possible mechanism of phytohormone perception in D14-mediated SL signalling. D14 functions as a cleavage enzyme of SLs, and the cleavage reaction induces the interaction with SLR1. The crystal structure of D14 shows that 5-hydroxy-3-methylbutenolide (D-OH), which is a reaction product of SLs, is trapped in the catalytic cavity of D14 to form an altered surface. The D14 residues recognizing D-OH are critical for the SL-dependent D14 À SLR1 interaction. These results provide new insight into crosstalk between gibberellin and SL signalling pathways.

Thalidomide and its derivatives exert not only therapeutic effects as immunomodulatory drugs (IMiDs) but also adverse effects such as teratogenicity, which are due in part to different C2H2 zinc-finger (ZF) transcription factors, IKZF1 (or IKZF3) and SALL4, respectively. Here, we report the structural bases for the SALL4-specific proteasomal degradation induced by 5-hydroxythalidomide, a primary thalidomide metabolite generated by the enzymatic activity of cytochrome P450 isozymes, through the interaction with cereblon (CRBN). The crystal structure of the metabolite-mediated human SALL4-CRBN complex and mutagenesis studies elucidate the complex formation enhanced by the interaction between CRBN and an additional hydroxy group of (S)-5-hydroxythalidomide and the variation in the second residue of β-hairpin structure that underlies the C2H2 ZF-type neo-morphic substrate (neosubstrate) selectivity of 5-hydroxythalidomide. These findings deepen our understanding of the pharmaceutical action of IMiDs and provide structural evidence that the glue-type E3 ligase modulators cause altered neosubstrate specificities through their metabolism.

FlAlyA is an endolytic enzyme with a preference for polymannuronate. The crystal structure and mutagenesis studies elucidated that the structural variations at outer uronate-binding subsites +2, +3 and -2 control the enzymatic properties of PL-7 family enzymes. Lys158 mutations changed the pH dependency and enhanced the production of mono- and disaccharides.

Human leukocyte cell-derived chemotaxin 2 (LECT2), which is predominantly expressed in the liver, is a multifunctional protein. LECT2 is becoming a potential therapeutic target for several diseases of worldwide concern such as rheumatoid arthritis, hepatocellular carcinoma, and obesity. Here, we present the crystal structure of LECT2, the first mammalian protein whose structure contains an M23 metalloendopeptidase fold. The LECT2 structure adopts a conserved Zn(II) coordination configuration but lacks a proposed catalytic histidine residue, and its potential substratebinding groove is blocked in the vicinity of the Zn(II)-binding site by an additional intrachain loop at the N terminus. Consistent with these structural features, LECT2 was found to be catalytically inactive as a metalloendopeptidase against various types of peptide sequences, including pentaglycine. In addition, a surface plasmon resonance analysis demonstrated that LECT2 bound to the c-Met receptor with micromolar affinity. These results indicate that LECT2 likely plays its critical roles by acting as a ligand for the corresponding protein receptors rather than as an enzymatically active peptidase. The intrachain loop together with the pseudoactive site groove in LECT2 structure may be specific for interactions between LECT2 and receptors. Our study reveals a mechanistic basis for the functional evolution of a mammalian protein with an M23 metalloendopeptidase fold and potentially broadens the implications for the biological importance of noncatalytic peptidases in the M23 family.Leukocyte cell-derived chemotaxin 2 (LECT2) 2 is a secretory protein originally identified as a chemotactic factor for neutrophils (1). LECT2 is predominantly expressed in the liver and is a direct target gene of Wnt/␤-catenin signaling (2-6). Accumulating evidence shows that mammalian LECT2 is a multifunctional protein that is closely associated with several diseases of worldwide concern, including hepatitis (7), rheumatoid arthritis (8 -10), hepatocellular carcinoma (HCC) (11, 12), obesity (13,14), and renal and hepatic amyloidosis (15-17). It has been reported that concanavalin A-induced hepatitis and collagen antibody-induced arthritis were suppressed by LECT2 (7,8). In addition, LECT2 expression inhibited the migration and invasion of human HCC cells in vitro and was negatively correlated with vascular invasion and tumor recurrence in HCC patients (11). In contrast, serum LECT2 levels were positively correlated with the severity of obesity and fatty liver in humans, and overproduction of LECT2 caused the development of obesity-associated insulin resistance (13,14). These findings suggest that LECT2 may be a candidate prognostic marker and a potential therapeutic target for these diseases. Despite the importance of LECT2 functions, the underlying mechanisms remain largely unclear.The mature human LECT2 is a basic protein consisting of 133 amino acids. Sequence similarity searches using BLAST have shown that LECT2 has a putative peptidase-M23 (PF01551) domain located ...