Wiley Emily A, Horrell Scott, Yoshino Alyssa, Schornak Cara C, Bagnani Claire, Chalker Douglas L
W.M. Keck Science Center of Claremont McKenna, Pitzer, and Scripps Colleges, Claremont, California, 91711.
Department of Biology, Washington University, St. Louis, Missouri, 63130.
J Eukaryot Microbiol. 2018 Jan;65(1):104-116. doi: 10.1111/jeu.12443. Epub 2017 Aug 3.
Proteins that possess a chromo domain are well-known for their roles in heterochromatin assembly and maintenance. The Heterochromatin Protein 1 (HP1) family, with a chromo domain and carboxy-terminal chromo shadow domain, targets heterochromatin through interaction with histone H3 methylated on lysine 9 (H3K9me2/3). The structural and functional diversity of these proteins observed in both fission yeast and metazoans correlate with chromatin specialization. To expand these studies, we examined chromo domain proteins in the ciliate Tetrahymena thermophila, which has functionally diverse and developmentally regulated heterochromatin domains. We identified thirteen proteins similar to HP1. Together they possess only a fraction of the possible chromo domain subtypes and most lack a recognizable chromo shadow domain. Using fluorescence microscopy to track chromatin localization of tagged proteins through the life cycle, we show evidence that in T. thermophila this family has diversified with biological roles in RNAi-directed DNA elimination, germline genome structure, and somatic heterochromatin. Those proteins with H3K27me3 binding sequence characteristics localize to chromatin in mature nuclei, whereas those with H3K9me2/3 binding characteristics localize to developing nuclei undergoing DNA elimination. Findings point to an expanded and diversified family of chromo domain proteins that parallels heterochromatin diversity in ciliates.
具有染色体结构域的蛋白质因其在异染色质组装和维持中的作用而闻名。异染色质蛋白1（HP1）家族具有一个染色体结构域和一个羧基末端染色体阴影结构域，通过与赖氨酸9甲基化的组蛋白H3（H3K9me2/3）相互作用靶向异染色质。在裂殖酵母和后生动物中观察到的这些蛋白质的结构和功能多样性与染色质特化相关。为了扩展这些研究，我们研究了嗜热四膜虫中的染色体结构域蛋白，嗜热四膜虫具有功能多样且受发育调控的异染色质结构域。我们鉴定出了13种与HP1相似的蛋白质。它们总共只拥有一部分可能的染色体结构域亚型，并且大多数缺乏可识别的染色体阴影结构域。通过荧光显微镜追踪标记蛋白在整个生命周期中的染色质定位，我们发现证据表明，在嗜热四膜虫中，这个家族已经多样化，在RNAi介导的DNA消除、生殖系基因组结构和体细胞异染色质中发挥生物学作用。那些具有H3K27me3结合序列特征的蛋白质定位于成熟细胞核中的染色质，而那些具有H3K9me2/3结合特征的蛋白质定位于正在进行DNA消除的发育中的细胞核。研究结果表明，染色体结构域蛋白家族已经扩展和多样化，这与纤毛虫中异染色质的多样性相平行。
