果蠅去頭蓋蛋白質2參與微管動態行為的調控

Abstract

轉譯抑制的訊息核醣蛋白質(mRNP)會和訊息核醣核酸(mRNA)結合聚集成 裂解體。裂解體內的蛋白質成員包含了訊息核醣核酸去頭蓋的複合體、去頭蓋蛋 白質的活化者、核醣核酸外切酶和其他等。去頭蓋蛋白質2(decapping protein 2) 則是負責在訊息核醣核酸去頭蓋的催化酵素。 在果蠅去頭蓋蛋白質2(Drosophila decapping protein 2)基因突變的卵母細胞 中，呈現不正常的肌動蛋白(actin)塊狀結構和細胞質流(ooplasmic streaming)缺 陷。由於細胞質流是一個依靠微管(microtubule)運作的行為，所以在果蠅去頭蓋 蛋白質2 基因突變的卵母細胞中，其細胞骨架與微管可能受到影響。 雖然果蠅去頭蓋蛋白質2 是一個去除頭蓋的酵素，且會參與訊息核醣核酸分 解；然而，在其突變的卵母細胞中所呈現的異常肌動蛋白塊狀結構，卻與肌動蛋 白基因的表現量無關。另一方面，在這些突變的卵母細胞中，除了細胞質流以外， 同時也觀察到與微管作用有關的性狀。第一，osk 訊息核醣核酸座落位置錯誤。 果蠅去頭蓋蛋白質1 (Drosophila decapping protein 1)是osk 訊息核醣核酸蛋白質 複合物之一員並主導osk 訊息核醣核酸的運輸；在果蠅去頭蓋蛋白質2 突變或秋 水仙素(colchicine)處理後的卵母細胞內，果蠅去頭蓋蛋白質1 仍會和錯位的osk 訊息核醣核酸座落在一起；此結果顯示果蠅去頭蓋蛋白質2 突變或秋水仙素處理 會影響卵母細胞內的微管，以至於運輸出現問題。第二，在野生型果蠅的卵腔， 第九期卵母細胞會表現微管梯度，第十期則呈現平行排列的維管束；但突變種的 微管梯度和微管束皆無法形成。第三，果蠅去蓋蛋白質2 突變的卵母細胞所表現 的微管崩解分散樣式，與野生型卵母細胞處理秋水仙素後，所展現的微管崩解樣 式非常類似。第四，為了要測試是否在果蠅去頭蓋蛋白質2 突變卵母細胞內，殘 存微管蛋白可以再重組成微管，於是加入了紫杉醇(taxol)促進微管蛋白結合並發 現有回復些許較細小的微管絲；也就是說，taxol 可部分抑制果蠅去頭蓋蛋白質2 突變所造成的影響。再者，雖然在果蠅去頭蓋蛋白質2 突變的幼蟲中，α-微管蛋白 (α-tubulin) 的蛋白質表現量下降，但是其訊息核醣核酸的表現量卻測不到明顯影響。 為了解果蠅去頭蓋蛋白質2在α/β-微管蛋白組成微管與微管間的動態平衡的角色，我們純化果蠅去頭蓋蛋白質2並做體外微管合成測試。果蠅去頭蓋蛋白質2的B表現形蛋白，雖然不像紫杉醇一樣可以影響到微管合成的階段，但是卻可維持平原期後的吸收曲線，而其他控制組的曲線已逐漸下降。更訝異的，在果蠅去頭蓋蛋白質2的A表現形蛋白中，扣除B表現形蛋白之外多出來的一段N端區域，也可以達到此現象。但是，具有催化活性以及核醣核酸結合區段的蛋白質，則對微管合成沒有影響。 在去除核糖核酸頭蓋的酵素功能外，果蠅去頭蓋蛋白質2可藉由刺激微管的合成或壓抑分解而調控微管的動態調控。

Processing bodies are aggregates of translationally repressed mRNPs associated with mRNA decay machinery. The protein components of P-bodies include mRNA decapping complex, activators of decapping protein, exonuclease and others. Decapping protein 2, Dcp2, is the catalytic enzyme of mRNA decapping. In dDcp2 mutant oocytes, they displayed abnormal actin clumps and defects in ooplasmic streaming. Since streaming is a microtubule-based action, the cytoskeletons abd microtubules in dDcp2 mutant oocytes might be affected. dDcp2 is a decapping enzyme participating mRNA decay; however, the ectopic actin clumps in dDcp2 mutant oocyte is not related to the quantity of actin gene expression. On the other hand, in dDcp2de21 mutant oocyte, several phenotypes related to microtubule function are observed besides ooplasmic streaming. First, the osk mRNA is mislocalized. dDcp1 which is a member of osk mRNP complex can direct the osk mRNA transport. In dDcp2 mutant or colchicine-treated oocyte, the mislocalized osk mRNA colocalizes with dDcp1. This result indicates that both dDcp2 mutation and colchicine treatment disturb microtubules in the oocyte, and in turn affecting the transportation of osk mRNA. Second, in wild-type egg chambers, microtubules display a gradient at stage 9 oocyte and a parallel array of microtubule bundles at stage 10 oocyte. However, both the gradient and bundles are disrupted in v dDcp2de21 mutant oocyte. Third, the microtubule expression pattern in dDcp2de21 mutant oocyte is very similar to the microtubule depolymerized situation when wild-type oocyte treated with colchicine. Fourth, to test whether the remaining tubulin in dDcp2de21 mutant oocyte could reorganize to microtubule, a narrow microtubule fibers is re-formed while the microtubule polymerizing drug, taxol, is added. That is, taoxl can partially suppress the phenotype caused by dDcp2 mutation. Further, though the expression of α-tubulin protein is decreased in dDcp2de21 mutant larvae, there is no significant deviation of tubulin mRNA level measured. Currently, we propose that dDcp2 is required for microtubule organization in Drosophila oocyte. In order to know the role of dDcp2 in dynamics between α/β tubulin heterodimers and microtubule, we purify dDcp2 protein to perform in vitro microtubule polymerization assay. dDcp2 B-form protein, which is not like taxol, can not influence the polymerization phase; however, it can maintain the absorbent curve from decreasing after plateau stage, while the curve has been gradually decreased in control protein. More strikingly, the unique N terminal domain in dDcp2A except for the dDcp2B can also achieve the situation. However, the protein containing catalytic and RNA-binding domain has no effect on microtubule polymerization. We suggest that, as a decapping enzyme, dDcp2 can participate in the regulation of microtubule dynamics by affecting the polymerization/depolymerization state.