Nuclear functions of the Arf guanine nucleotide exchange factor BRAG2
Nuclear functions of the Arf guanine nucleotide exchange factor BRAG2
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DOI：
10.1111/j.1600-0854.2007.00561.x
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发表时间：
2007-06-01
期刊：
影响因子：
4.5
通讯作者：
Casanova, James E.
中科院分区：
文献类型：
--
作者：
Dunphy, Jillian L.;Ye, Keqiang;Casanova, James E.
BRAG 2是GT3 Arf 6的鸟嘌呤核苷酸交换因子，GT3 Arf 6以CRM 1/exportin 1依赖性方式在细胞质和细胞核之间循环。尽管它存在于细胞核中，但以前没有描述过核功能。在这里，我们表明，通过RNAi耗尽内源性BRAG 2导致Cajal小体（CB）数量增加，核仁结构改变，如较少的局灶性纤维蛋白染色所示。考虑到核BRAG 2在整个核质中弥散分布并且在稳态下不集中在核仁内，该结果是令人惊讶的。然而，我们发现核GTIPIKE/AGAP 2的异位表达导致BRAG 2和CB标记物卷曲蛋白在核仁中积累。PIKE的GTP酶活性和BRAG 2的核苷酸交换活性都不是这种核仁浓度所必需的。核仁中外源性BRAG 2水平的增加导致原纤蛋白向核仁外周的重新分布，支持BRAG 2在调节核仁结构中的作用。这些观察结果表明，除了其在质膜的内吞调节中的作用外，BRAG 2还在细胞核内发挥作用。
BRAG2 is a guanine nucleotide exchange factor for the GTPase Arf6 that cycles between the cytoplasm and nucleus in a CRM1/exportin1-dependent manner. Despite its presence in the nucleus, nuclear functions have not previously been described. Here, we show that depletion of endogenous BRAG2 by RNAi leads to an increased number of Cajal bodies (CBs), and altered structure of nucleoli, as indicated by less focal fibrillarin staining. This result was surprising given that nuclear BRAG2 is diffusely distributed throughout the nucleoplasm and is not concentrated within nucleoli at steady state. However, we found that ectopic expression of the nuclear GTPase PIKE/AGAP2 causes both BRAG2 and the CB marker coilin to accumulate in nucleoli. Neither the GTPase activity of PIKE nor the nucleotide exchange activity of BRAG2 is required for this nucleolar concentration. Increased levels of exogenous BRAG2 in nucleoli result in a redistribution of fibrillarin to the nucleolar periphery, supporting a role for BRAG2 in regulating nucleolar architecture. These observations suggest that, in addition to its role in endocytic regulation at the plasma membrane, BRAG2 also functions within the nucleus.