The histone locus body (HLB) is a membraneless organelle that determines the transcription of replication-dependent histones. However, the mechanisms underlying the appropriate formation of the HLB in the nucleus but not in the cytoplasm remain unknown. HLB formation is dependent on the scaffold protein NPAT. We identify KPNA3 as a specific importin that drives the nuclear import of NPAT by binding to the nuclear localization signal (NLS) sequence. NPAT undergoes phase separation, which is inhibited by KPNA3-mediated impairment of self-association. In this, a C-terminal self-interaction facilitator (C-SIF) motif, proximal to the NLS, binds the middle 431-1,030 sequence to mediate the self-association of NPAT. Mechanistically, the anchoring of KPNA3 to the NPAT-NLS sterically blocks C-SIF motif-dependent NPAT self-association. This leads to the suppression of aberrant NPAT condensation in the cytoplasm. Collectively, our study reveals a previously unappreciated role of KPNA3 in modulating HLB formation and delineates a steric hindrance mechanism that prevents inappropriate cytoplasmic NPAT condensation.

中文翻译：

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KPNA3 通过调节 NPAT 的凝聚和核输入来调节组蛋白位点体的形成。


组蛋白位点体 （HLB） 是一种无膜细胞器，可决定复制依赖性组蛋白的转录。然而，HLB 在细胞核中而不是在细胞质中适当形成的机制仍然未知。HLB 的形成取决于支架蛋白 NPAT。我们将 KPNA3 鉴定为一种特异性输入蛋白，它通过与核定位信号 （NLS） 序列结合来驱动 NPAT 的核输入。NPAT 发生相分离，这被 KPNA3 介导的自缔合损伤所抑制。在这种情况下，NLS 近端的 C 端自相互作用促进剂 （C-SIF） 基序结合中间的 431-1,030 序列以介导 NPAT 的自结合。从机制上讲，KPNA3 与 NPAT-NLS 的锚定空间阻断了 C-SIF 基序依赖性 NPAT 自缔合。这导致细胞质中异常的 NPAT 浓缩受到抑制。总的来说，我们的研究揭示了 KPNA3 在调节 HLB 形成中以前未被重视的作用，并描绘了防止不适当的细胞质 NPAT 凝聚的空间位阻机制。