目前普遍認(rèn)為,，選擇性剪接(alternative splicing)是機體增加蛋白多樣性的一種有效方式，也是機體調(diào)控蛋白表達(dá)的一種重要機制,。在人類和靈長類動物中,，保守估計約有75%的基因發(fā)生選擇性剪接，尤其多發(fā)生在免疫系統(tǒng),。一個高效的免疫系統(tǒng)必須適時和適當(dāng)?shù)貙Σ≡⑸镒鞒雒庖邞?yīng)答,，而選擇性剪接就是免疫系統(tǒng)發(fā)揮其正常生物學(xué)功能而有效利用的一種機制。
主要組織相容性復(fù)合物(Major histocompatibility complex,， MHC)是廣泛存在于脊椎動物體內(nèi),、與免疫功能密切相關(guān)的一組基因群，其基因產(chǎn)物不僅參與移植排斥和T細(xì)胞的分化發(fā)育,，在免疫應(yīng)答的啟動和免疫調(diào)節(jié)中也發(fā)揮重要作用,。眾多研究表明，MHC I類分子(MHC I)在不同物種中均會發(fā)生不同程度的選擇性剪接,，產(chǎn)生新的剪接異構(gòu)體,。但到目前為止，對MHC I類分子選擇性剪接的研究往往僅停留在鑒別新的MHC異構(gòu)體上,，而對其在免疫系統(tǒng)中精細(xì)調(diào)控卻知之甚少,。
為探討剪接異構(gòu)體對全長型MHC IA分子的影響以及在免疫調(diào)控中的作用，中國科學(xué)院昆明動物研究所動物模型與人類疾病機理重點實驗室博士生戴正喜等人在導(dǎo)師鄭永唐研究員的指導(dǎo)下,，以人類近親獼猴為模型動物,，在獼猴外周血單個核細(xì)胞中發(fā)現(xiàn)和鑒定了一種新的MHC IA剪接異構(gòu)體，命名為MHC IA-sv1,。通過運用生物化學(xué)和分子生物學(xué)實驗方法,，研究發(fā)現(xiàn)MHC I A-sv1雖缺少α3結(jié)構(gòu)域，但卻可以表達(dá)于細(xì)胞膜表面,。另外，MHC IA-sv1的糖基化模式和蛋白降解速度明顯異于全長型的MHC IA分子,。
特別有趣的是,，MHC IA-sv1能與MHC IA形成全新的異源二聚體結(jié)構(gòu)，此結(jié)構(gòu)不再結(jié)合β2微球蛋白,。進(jìn)一步研究還表明,，此異源復(fù)合體能顯著地抑制MHC IA蛋白的泛素化，從而促進(jìn)其蛋白的穩(wěn)定性,。
這種新的順式調(diào)控模式為人們理解MHC I剪接異構(gòu)體對免疫應(yīng)答的影響提供了一種全新的視角,。
國際同行評審專家認(rèn)為該研究結(jié)果是MHC剪接領(lǐng)域的一項重要進(jìn)展。相關(guān)研究論文目前已發(fā)表于國際學(xué)術(shù)期刊《免疫學(xué)雜志》Journal of Immunology,。該項目研究得到了國家自然科學(xué)基金的經(jīng)費資助,。（生物谷Bioon.com）
doi:10.4049/jimmunol.1100665
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The β2-Microglobulin–Free Heterodimerization of Rhesus Monkey MHC Class I A with Its Normally Spliced Variant Reduces the Ubiquitin-Dependent Degradation of MHC Class I A
Zheng-Xi Dai*,†, Gao-Hong Zhang*, Xi-He Zhang*,†, Hou-Jun Xia*,†, Shao-You Li*,† and Yong-Tang Zheng*
The MHC class I (MHC I) molecules play a pivotal role in the regulation of immune responses by presenting antigenic peptides to CTLs and by regulating cytolytic activities of NK cells. In this article, we show that MHC I A in rhesus macaques can be alternatively spliced, generating a novel MHC I A isoform (termed “MHC I A-sv1”) devoid of α3 domain. Despite the absence of β2-microglobulin (β2m), the MHC I A-sv1 proteins reached the cell surface of K562-transfected cells as endoglycosidase H-sensitive glycoproteins that could form disulfide-bonded homodimers. Cycloheximide-based protein chase experiments showed that the MHC I A-sv1 proteins were more stable than the full-length MHC I A in transiently or stably transfected cell lines. Of particular interest, our studies demonstrated that MHC I A-sv1 could form β2m-free heterodimers with its full-length protein in mammalian cells. The formation of heterodimers was accompanied by a reduction in full-length MHC I A ubiquitination and consequent stabilization of the protein. Taken together, these results demonstrated that MHC I A-sv1 and MHC I A can form a novel heterodimeric complex as a result of the displacement of β2m and illustrated the relevance of regulated MHC I A protein degradation in the β2m-free heterodimerization-dependent control, which may have some implications for the MHC I A splice variant in the fine tuning of classical MHC I A/TCR and MHC I A/killer cell Ig-like receptor interactions.
