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Induction of selective degradation of target proteins by small molecules (protein knockdown) would be useful for biological research and treatment of various diseases. To achieve protein knockdown, we utilized the ubiquitin ligase activity of cellular inhibitor of apoptosis protein 1 (cIAP1), which is activated by methyl bestatin (MeBS, 2). We speculated that formation of an artificial (nonphysiological) complex of cIAP1 and a target protein would be induced by a hybrid molecule consisting of MeBS (2) linked to a ligand of the target protein, and this would lead to cIAP1-mediated ubiquitination and subsequent proteasomal degradation of the target protein. To verify this hypothesis, we focused on cellular retinoic acid-binding proteins (CRABP-I and -II) and designed hybrid molecules (compounds 4) consisting of MeBS (2) coupled via spacers of various lengths to all-trans retinoic acid (ATRA, 3), a ligand of CRABPs. Compounds 4 induced selective loss of CRABP-I and -II proteins in cells. We confirmed that 4b induced formation of a complex of cIAP1 and CRABP-II in vitro and induced proteasomal degradation of CRABP-II in cells. When neuroblastoma IMR-32 cells were treated with 4b, the level of CRABP-II was reduced and cell migration was inhibited, suggesting potential value of CRABP-II-targeting therapy for controlling tumor metastasis. Our results indicate that 4b possesses sufficient activity, permeability, and stability in cells to be employed in cellular assays. Hybrid molecules such as 4 should be useful not only as chemical tools for studying the biological/physiological functions of CRABPs but also as candidate therapeutic agents targeting CRABPs.

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Edited by Noboru MizushimaKeywords: Cellular inhibitor of apoptosis protein 1 Cellular retinoic acid binding protein-II Protein knockdown Ubiquitin Proteasome a b s t r a c t Manipulation of protein stability with small molecules is a challenge in the field of drug discovery. Here we show that cellular retinoic acid binding protein-II (CRABP-II) can be specifically degraded by a novel compound, SNIPER-4, consisting of (À)-N-[(2S,3R)-3-amino-2-hydroxy-4-phenylbutyryl]-L-leucine methyl ester and all-trans retinoic acid that are ligands for cellular inhibitor of apoptosis protein 1 (cIAP1) and CRABP-II, respectively. Mechanistic analysis revealed that SNIPER-4 induces cIAP1-mediated ubiquitylation of CRABP-II, resulting in the proteasomal degradation. The protein knockdown strategy employing the structure of SNIPER-4 could be applicable to other target proteins.

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While electrophilic reagents for histidine labeling have been developed, we report an umpolung strategy for histidine functionalization. A nucleophilic small molecule, 1-methyl-4-arylurazole, selectively labeled histidine under singlet oxygen (1O2) generation conditions. Rapid histidine labeling can be applied for instant protein labeling. Utilizing the short diffusion distance of 1O2 and a technique to localize the 1O2 generator, a photocatalyst in close proximity to the ligand-binding site, we demonstrated antibody Fc-selective labeling on magnetic beads functionalized with a ruthenium photocatalyst and Fc ligand, ApA. Three histidine residues located around the ApA binding site were identified as labeling sites by liquid chromatography–mass spectrometry analysis. This result suggests that 1O2-mediated histidine labeling can be applied to a proximity labeling reaction on the nanometer scale.

Parkin ubiquitin (Ub) ligase (also known as PARK2) ubiquitinates damaged mitochondria for their clearance and quality control. USP30 deubiquitinase opposes parkin-mediated Ub-chain formation on mitochondria by preferentially cleaving Lys6-linked Ub chains. Here, we report the crystal structure of zebrafish USP30 in complex with a Lys6-linked diubiquitin (diUb or Ub) at 1.87-Å resolution. The distal Ub-recognition mechanism of USP30 is similar to those of other USP family members, whereas Phe4 and Thr12 of the proximal Ub are recognized by a USP30-specific surface. Structure-based mutagenesis showed that the interface with the proximal Ub is critical for the specific cleavage of Lys6-linked Ub chains, together with the noncanonical catalytic triad composed of Cys-His-Ser. The structural findings presented here reveal a mechanism for Lys6-linkage-specific deubiquitination.

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