研究発表

FOXO3a is a transcription activator of apoptosis-related genes [1]. In cancer cells, FOXO3a is phosphorylated by a Ser/Thr kinase AKT, and then recognized by an adopter protein 14-3-3ζ [1]. The 14-3-3ζ binding dissociates FOXO3a from DNA, which suppresses the transcriptional activity [1, 2]. Consequently, apoptosis of the cancer cells is impaired, resulting in the cancer exacerbation. Although the interaction between 14-3-3ζ and FOXO3a is a potential target for anti-cancer drugs, it remains unknown how 14-3-3ζ binds to FOXO3a and dissociates it from DNA. To reveal this mechanism, we investigated the interaction FOXO3a with DNA and 14-3-3ζ. We prepared human 14-3-3ζ as a functional dimer, and human FOXO3a residues 1-284 containing a DNA binding domain (DBD) as well as two 14-3-3ζ-binding motifs, Np and Cp, each of which possess a phosphorylation site, T32 and S253, respectively. Di-phosphorylated form of FOXO3a (dpFOXO3a) was prepared by phosphorylation using AKT. Isothermal titration calorimetry (ITC) analyses showed that the dissociation constant (Kd) for the dpFOXO3a-14-3-3ζ binding was 59 nM and that for the dpFOXO3a-DNA binding was 131 nM. Although their Kd values differed only 2-fold, fluorescence-detection size exclusion chromatography (FSEC) indicated that 95% of DNA bound to dpFOXO3a was substituted by 14-3-3ζ, when 1 equivalent of 14-3-3ζ dimer was added. NMR analyses demonstrated that, while 14-3-3ζ mainly interacts with Np and Cp of dpFOXO3a, it directly interacts with the DNA-binding surface on DBD of dpFOXO3a, indicating that the replacement of DNA with 14-3-3ζ is the direct competition. However, the binding affinity of 14-3-3ζ for the isolated DBD is 700-fold weaker than that of DNA, suggesting that the tethering via Np and Cp should plays an important role in the replacement of DNA. FSEC analyses with mono-phosphorylated form of the FOXO3a mutants demonstrated that 14-3-3ζ binding to N-pFOXO3a (S253A) – DNA complex did not dissociate DNA, whereas 14-3-3ζ binding to C-pFOXO3a (T32A) – DNA complex dissociates 42% of DNA from C-pFOXO3a (T32A). These results together with the FSEC results of dpFOXO3a indicated that the 14-3-3ζ binding to Cp is more necessary to dissociate DNA, and both Np and Cp are required for the complete DNA dissociation. Detailed analysis of the NMR spectra revealed that the DBD of dpFOXO3a bound and traveled to each protomer of the 14-3-3ζ dimer. These results suggest that efficient substitution of DNA with14-3-3ζ occurs by their competition on the DBD, in which direct but weak binding of 14-3-3ζ to the DBD is significantly enhanced by increase in the accessibility and the local concentration of 14-3-3ζ around the DBD through tethering via the Np and Cp.

新型コロナウイルス感染症（COVID-19）の原因となるSARS-CoV-2ウイルスは、そのスパイクタンパク質を用いてヒト細胞表面にあるアンジオテンシン変換酵素2（ACE2）受容体と相互作用することで細胞内に侵入する。また、ウイルスに変異が入りやすく、それによる変異株は免疫を逃避することが課題であるが、現状ウイルスの変異に対応できる治療薬は存在しない。したがって、宿主の受容体であるACE2を標的としてスパイクタンパク質との結合を阻害する化合物はウイルスの変異に対応可能な治療薬となり得る。そこで本研究では、当研究室で開発された「標的タンパク質指向型天然物単離法」としてACE2ビーズ法を構築し、沖縄県独自の植物エキスライブラリーからACE2に結合する天然物を探索することを目的とする。

新型コロナウイルス感染症（COVID-19）は、2019年暮れに中国・武漢で世界で初めて患者が確認されてから、2ヶ月あまりで世界中に拡散し、世界保健機構（WHO）も2020年3月にパンデミックを宣言した。COVID-19は季節性インフルエンザや新型インフルエンザに比べて感染力が高く1，2)、感染により肺などの呼吸器系だけではなく、特に心臓などの循環器系の合併症や様々な罹患後症状も引き起こす3， 4)。また、COVID-19の原因ウイルスであるSARS-CoV-2には変異が入りやすく、それによる変異株は自然感染やワクチンによって獲得した免疫を逃避することが報告されている5)。現在承認されているSARS-CoV-2の抗ウイルス薬はウイルスを標的としており、ウイルス変異によりその有効性が失われる可能性がある。そのため、未だにSARS-CoV-2の変異に対応できる治療薬の開発が求められている。

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by the SARS-CoV-2 virus. Although several treatments and vaccines are available, definitive treatment has yet to be established. Since the interaction between the receptor binding domain (RBD) of SARS-CoV-2 spike protein and the extracellular domain of hACE2 is important for viral entry, compounds that directly bind to either the RBD or hACE2 and inhibit their interaction should be a drug candidate for COVID-19. Here, we performed ELISA-based assays to find entry inhibitors targeting the interaction between the RBD and hACE2 using small-to-medium sized compounds from our PPI-inhibitor library, DLiP [1], and a number of compounds proposed by in silico approach. The assays discovered multiple hit candidates and most of them inhibited the SARS-CoV-2 infection confirmed by viral infection assay, however the estimated IC50 values were ~10-4 M. The NMR-derived information on the binding sites on the protein would be useful for drug design to improve binding affinity and thus inhibitory activity of the candidates. Our NMR analyses indicated that some compounds, possessing common chemical structure, directly bound to hACE2. To identify their binding site on hACE2 and design the compounds with higher affinity, assignments of the NMR signals of hACE2 are underway.