BCAS2為果蠅發育及哺乳類細胞生長之重要蛋白質

Abstract

BCAS2( Breast cancer amplification sequence 2 )是一個功能未明的核蛋白，位於染色體1p13.3-21 的位置，轉譯出225 個胺基酸組成的蛋白質產物，在人類組織中普遍表現。 BCAS2 在物種間具有高度保留性，人類BCAS2 的蛋白質序列與黑猩猩、狗、老鼠等哺乳類動物的相似度高達100%。本實驗室發現果蠅體內的基因CG4980 與人類BCAS2 相似度達59%，主要差異為CG4980 的N 端多了66 個胺基酸，顯示CG4980 為BCAS2 在果蠅內的同源基因。利用果蠅做為默化BCAS2的動物模式，我們發現缺乏BCAS2 會使果蠅出現胸節及翅膀的發育異常；外送入人類BCAS2 則可以挽救發育異常的形態。說明CG4980 與BCAS2 在生物體中功能相似，且對於果蠅的發育而言扮演著重要的角色。 BCAS2 在本實驗室的研究中，發現可以與抑癌基因p53 結合，抑制p53 的活性，默化BCAS2 會使具有p53 的細胞凋亡。本研究中則發現，在無p53 或p53 突變的細胞中默化BCAS2，不會造成細胞凋亡，但卻會發生細胞週期停滯在G2/M期。此外，在人類細胞中默化BCAS2 會造成細胞出現DNA 損傷的現象，DNA 損傷的標誌－γH2AX 會顯著上升。表示BCAS2 對於人類細胞的生長而言似乎是必須的。本研究也證明了BCAS2 藉由蛋白質N 端與Pso4 結合，參與在Pso4/Prp19 複 合體中，此複合體在酵母菌及人類細胞內都被報導出具有參與mRNA 剪切及DNA 修復的功能。我們利用in vivo 證實BCAS2 具有初mRNA 剪切的功能，也觀察到BCAS2 可以協助細胞抵抗外力造成的DNA 雙股斷裂，降低DNA 損傷反應。然而BCAS2 參與在剪切與修復功能中的詳細作用機轉仍尚未釐清。 本研究說明了BCAS2 是一個雙功能的蛋白質，它負責調控的功能與細胞的生長、發育有密切的關係，缺乏BCAS2 會造成細胞與個體的異常。未來可以此探討，BCAS2 的缺乏是否為疾病或癌症的指標之ㄧ。

BCAS2 ( Breast cancer amplification sequence 2 ) is a novel gene locate on the chromosome 1p13.3-21 region and ubiquitously expressed in many human organs. BCAS2 cDNA contained a full coding region of 678 bp encoding 225 amino acids with the predicted molecular mass of 26 kDa. BCAS2 shows highly conserved in vertebrate. The predicted peptide sequence of the BCAS2 gene shows 100% homology to Pan troglodytes, Canis familiaris, Mus musculus and Rattus norvegicus. In Drosophila melanogaster, we find that CG4980 is the homologene of human BCAS2. Accordingly, we use D. melanogaster as the animal model to investigate the function of BCAS2. We find that silenced CG4980 leads damaging phenotype on thorax and wing in D. melanogaster, and human BCAS2 overexpression can rescue the damage. Consequently, human BCAS2 plays a similar role to CG4980, and it seems CG4980 is an essential gene for fly development. Besides, BCAS2 is also important to human cell proliferation. Our lab has demonstrated that depletion of BCAS2 by siRNA can enhance p53 activity and cause cell apoptosis. In this study, silencing of BCAS2 in p53-null or p53-mutant cell lines results in cell cycle arrest at G2/M and DNA damage, increasing the expression of γH2AX, a marker for DNA damage. Then we find that BCAS2 can interact with Pso4 by N terminal. It demonstrates that BCAS2 indeed involve in Pso4/Prp19 complex. Pso4/Prp19 complex is identified as a splicing complex, plays an important role in pre-mRNA splicing. Besides, Pso4/Prp19 complex also can regulate some type of DNA repair. We find that BCAS2 shows pre-mRNA splicing activity by in vivo splicing assay. Moreover, while treated with DNA damage matter, BCAS2 overexpression can assist cells to resist DNA damage and down regulate the expression of γH2AX. Together with our finding, BCAS2 may be a bifunctional protein, with independent function in RNA splicing and DNA repair, and play important roles in regulating organism development and cell proliferation.