Padavattan Sivaraman, Thiruselvam Viswanathan, Shinagawa Toshie, Hasegawa Kazuya, Kumasaka Takashi, Ishii Shunsuke, Kumarevel Thirumananseri
RIKEN SPring-8 Center, Harima Institute, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan.
Centre of Advanced Study in Crystallography and Biophysics, University of Madras, Guindy Campus, Chennai 600 025, India.
Biophys Chem. 2017 Feb;221:41-48. doi: 10.1016/j.bpc.2016.11.013. Epub 2016 Dec 5.
Th2a and Th2b are the testis-specific histone variants highly expressed during spermatogenesis. Approximately 4% of the genome is retained in nucleosomes in mature human sperm, which is enriched at loci of developmental importance. Our recent studies revealed that the mouse histone variant homologs TH2a and TH2b are involved in reprogramming. In the present work, we report three nucleosome structures (NCPs) with human testis-specific histone variants hTh2a and hTh2b, [hGcH (hTh2a-hTh2b-H3-H4), hGcHV1 (hTh2a-H2b-H3-H4) and hGcHV2 (H2a-hTh2b-H3-H4)] and a 146-base pair (bp) duplex DNA fragment at ~3.0Å resolutions. These crystal structures revealed two major changes within the nucleosomes, either with hTh2a, hTh2b or both variants, as compared to the canonical counterpart. First, the H-bonding interactions between the L1-L1' interfaces mediated by the hTh2a/hTh2a' L1-loops are lost. Second, the histone dimer-DNA contacts are considerably reduced, and these changes are localized around ±31 to 35-bp from the nucleosome entry/exit sites. Thus, the modified functional residues at the N- and C-terminal ends of histone variants are responsible for the observed structural changes and regulate the gene expression through specific structural alterations in the chromatin by modulating the chromatin-associated binding proteins.
Th2a和Th2b是在精子发生过程中高度表达的睾丸特异性组蛋白变体。在成熟人类精子中，约4%的基因组保留在核小体中，这些核小体在具有发育重要性的基因座处富集。我们最近的研究表明，小鼠组蛋白变体同源物TH2a和TH2b参与重编程。在本研究中，我们报告了三种具有人类睾丸特异性组蛋白变体hTh2a和hTh2b的核小体结构（核小体核心颗粒，NCPs），[hGcH（hTh2a-hTh2b-H3-H4）、hGcHV1（hTh2a-H2b-H3-H4）和hGcHV2（H2a-hTh2b-H3-H4）]以及一个146碱基对（bp）的双链DNA片段，分辨率约为3.0Å。这些晶体结构揭示了与经典对应物相比，核小体内的两个主要变化，要么是hTh2a、hTh2b，要么是两者都有变体。首先，由hTh2a/hTh2a' L1环介导的L1-L1'界面之间的氢键相互作用丧失。其次，组蛋白二聚体与DNA的接触显著减少，这些变化位于距核小体进出位点约±31至35bp处。因此，组蛋白变体N端和C端的修饰功能残基负责观察到的结构变化，并通过调节与染色质相关的结合蛋白，通过染色质中的特定结构改变来调控基因表达。
