Sm protein methylation is dispensable for snRNP assembly in Drosophila melanogaster
Sm protein methylation is dispensable for snRNP assembly in Drosophila melanogaster
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DOI：
10.1261/rna.940708
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发表时间：
2008-05-01
期刊：
影响因子：
4.5
通讯作者：
Matera, A. Gregory
中科院分区：
文献类型：
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作者：
Gonsalvez, Graydon B.;Praveen, Kavita;Matera, A. Gregory
Sm蛋白与小核(Sn)RNA形成稳定的核糖核蛋白(RNP)复合体，是真核剪接体的核心成分。在体内，Sm蛋白组装到SnRNAs上需要存活运动神经元(SMN)复合体。一些报道表明，SMN蛋白与SMB、SmD1和SmD3的C-末端存在的对称二甲基精氨酸(SDMA)残基具有高亲和力。这种翻译后修饰被认为在SnRNP组装中起着至关重要的作用。在人类细胞中，两种不同的蛋白质精氨酸甲基转移酶(PRMT5和PRMT7)是SNRNP生物发生所必需的。然而，在果蝇中，Dart5(果蝇PRMT5的同源基因)的缺失对SnRNP的组装几乎没有影响，纯合突变是完全可行的。为了解决这些明显的差异，我们详细研究了这个话题，发现果蝇Sm蛋白也被两个甲基转移酶Dart5/PRMT5和Dart7/PRMT7甲基化。与DART5不同的是，我们发现DART7是一个必不可少的基因。然而，与Dart7蛋白丢失相关的致命性显然与SnRNP组装的缺陷无关。为了最终测试果蝇SnRNP组装中Sm蛋白SDMA修饰的需求，我们构建了一个仅表达SmD1亚型的果蝇菌株，该亚型不能被SDMA修饰。有趣的是，这些果蝇是存活的，而且与野生型相比，SnRNP检测显示没有缺陷。相反，在存在SMN亚型突变的情况下，DART5突变体表现出强烈的合成致死表型。因此，我们得出结论，当SMN是有限的时，DART5是生存所必需的。
Sm proteins form stable ribonucleoprotein (RNP) complexes with small nuclear (sn) RNAs and are core components of the eukaryotic spliceosome. In vivo, the assembly of Sm proteins onto snRNAs requires the survival motor neurons (SMN) complex. Several reports have shown that SMN protein binds with high affinity to symmetric dimethylarginine (sDMA) residues present on the C-terminal tails of SmB, SmD1, and SmD3. This post-translational modification is thought to play a crucial role in snRNP assembly. In human cells, two distinct protein arginine methyltransferases (PRMT5 and PRMT7) are required for snRNP biogenesis. However, in Drosophila, loss of Dart5 (the fruit fly PRMT5 ortholog) has little effect on snRNP assembly, and homozygous mutants are completely viable. To resolve these apparent differences, we examined this topic in detail and found that Drosophila Sm proteins are also methylated by two methyltransferases, Dart5/PRMT5 and Dart7/PRMT7. Unlike dart5, we found that dart7 is an essential gene. However, the lethality associated with loss of Dart7 protein is apparently unrelated to defects in snRNP assembly. To conclusively test the requirement for sDMA modification of Sm proteins in Drosophila snRNP assembly, we constructed a fly strain that exclusively expresses an isoform of SmD1 that cannot be sDMA modified. Interestingly, these flies were viable, and snRNP assays revealed no defects in comparison to wild type. In contrast, dart5 mutants displayed a strong synthetic lethal phenotype in the presence of a hypomorphic Smn mutation. We therefore conclude that dart5 is required for viability when SMN is limiting.