Department of Bioactive Molecules, National Institute of Infectious Diseases, 1–23–1 Toyama, Tokyo162–8640, Japan.
Biol Pharm Bull. 2012;35(11):2064-8. doi: 10.1248/bpb.b12-00527. Epub 2012 Sep 7.
The bromodomain and extraterminal (BET) family is a group of chromatin-binding proteins characterized by two bromodomains, an extraterminal (ET) domain, and several other conserved regions of unknown function. In humans, the BET family consists of four members, BRD2, BRD3, BRD4 and BRDT, that all normally localize to the nucleus. We identified a 12-amino acid stretch in the inter-bromodomain region that is perfectly conserved among the BET family members. We deleted these residues and expressed the mutant proteins in HEK293T cells to investigate the function of this motif. We found that the deletion of this motif alters the localization of BET proteins. Mutated BRD3 and BRD4 were excluded from the nucleus, and BRDT was found to be diffused throughout the nucleus and cytoplasm. Although the mutant BRD2 remained predominantly in the nucleus, a punctate distribution was also observed in the cytosol. It has been reported that a conserved motif between the second bromodomain and the ET domain serves as a nuclear localization signal for BRD2. Nevertheless, BET mutants lacking the reported nuclear localization signal motif but retaining the 12-amino acid stretch resided in the nucleus. Furthermore, these mutants were diffused throughout the cytoplasm when the 12 residues were removed. These results indicate that the conserved amino acid stretch in the inter-bromodomain region of the BET family functions as a nuclear localization signal.
溴结构域和末端（BET）家族是一组染色质结合蛋白，其特征是两个溴结构域、一个末端（ET）结构域和几个其他未知功能的保守区域。在人类中，BET 家族由四个成员组成，BRD2、BRD3、BRD4 和 BRDT，它们都正常定位于细胞核内。我们在溴结构域之间发现了一段 12 个氨基酸的延伸，该延伸在 BET 家族成员中完全保守。我们删除了这些残基，并在 HEK293T 细胞中表达突变蛋白，以研究该模体的功能。我们发现，该模体的缺失改变了 BET 蛋白的定位。突变的 BRD3 和 BRD4 被排除在细胞核外，而 BRDT 则被发现弥散在细胞核和细胞质中。尽管突变的 BRD2 主要仍位于细胞核内，但在细胞质中也观察到点状分布。据报道，第二个溴结构域和 ET 结构域之间的保守模体是 BRD2 的核定位信号。然而，缺失报道的核定位信号模体但保留 12 个氨基酸延伸的 BET 突变体仍位于细胞核内。此外，当去除这 12 个残基时，这些突变体弥散到整个细胞质中。这些结果表明，BET 家族溴结构域之间的保守氨基酸延伸作为核定位信号发挥作用。
