Purified U7 snRNPs lack the Sm proteins D1 and D2 but contain Lsm10, a new 14 kDa Sm D1-like protein

U7 snRNPs were isolated from HeLa cells by biochemical fractionation, followed by affinity purification with a biotinylated oligonucleotide complementary to U7 snRNA. Purified U7 snRNPs lack the Sm proteins D1 and D2, but contain additional polypeptides of 14, 50 and 70 kDa. Microsequencing identified the 14 kDa polypeptide as a new Sm-like protein related to Sm D1 and D3. Like U7 snRNA, this protein, named Lsm10, is enriched in Cajal bodies of the cell nucleus. Its incorporation into U7 snRNPs is largely dictated by the special Sm binding site of U7 snRNA. This novel type of Sm complex, composed of both conventional Sm proteins and the Sm-like Lsm10, is most likely to be important for U7 snRNP function and subcellular localization.

研究人员通过生化分级分离法从海拉细胞中分离得到U7小核糖核蛋白颗粒（U7 snRNPs），随后采用与U7 snRNA互补的生物素标记寡核苷酸进行亲和纯化。纯化获得的U7 snRNPs缺失Sm蛋白D1与D2，但含有分子量分别为14 kDa、50 kDa与70 kDa的额外多肽。通过微测序技术鉴定，该14 kDa多肽为一类与Sm D1、D3同源的新型类Sm蛋白，将其命名为Lsm10。与U7 snRNA一致，该蛋白在细胞核内的卡哈尔体（Cajal bodies）中富集。Lsm10整合进入U7 snRNPs的过程主要由U7 snRNA的特殊Sm结合位点所介导。这种由传统Sm蛋白与类Sm蛋白Lsm10共同组成的新型Sm复合物，极有可能在U7 snRNPs的功能发挥及亚细胞定位中发挥关键作用。