Live-cell imaging of HP1α throughout the cell cycle of mouse C3H10T1/2 cells and rhythmical flickering of heterochromatin dots in interphase

Abstract

Heterochromatin protein 1 alpha (HP1α) localizes to heterochromatin in interphase and shows dynamic molecular behavior in living cells. We previously reported that during mitosis, the majority of HP1α diffused into the cytoplasm but some remained in centromere heterochromatin. Here, we further characterize the molecular behavior of HP1α throughout the cell cycle. Time-lapse imaging of DsRed-HP1α through two successive cell divisions indicated that interphase can be divided into four phases. HP1α forms heterochromatin dots in early G1, which are maintained without any apparent changes (Phase 1). However, the HP1α dots begin to diffuse into the nucleoplasm and start flickering with a rhythmical cycle (Phase 2). Then, the HP1α dots diffuse further towards the periphery of the nucleus (Phase 3), and uniformly diffuse throughout the entire nucleus (Phase 4). Rhythmical flickering of HP1α dots in the middle of interphase may be useful for following cell cycle progression in mouse living cells.

RNA chain length and stoichiometry govern surface tension and stability of protein-RNA condensates

“…The functional roles of condensates are yet to be fully elucidated. On the grounds of thermodynamic and kinetic control of phase transitions, however, many studies have already proposed functions of condensates which encompass transcriptional noise buffering ( Simpson-Lavy and Kupiec, 2021 ; Riback et al., 2020 ), enhancement of enzymatic catalysis ( O’Flynn and Mittag, 2021 ; Nakashima et al., 2021 ; Peeples and Rosen, 2021 ), and formation of heterochromatin domains in the nucleus ( Rippe, 2021 ; Machida et al., 2018 ; Nakagawa et al., 2014 ). When it comes to possible gene-regulatory functions of condensation, RNA molecules are very likely to play significant roles because proteomic studies find ubiquitous enrichment of RNAs and RNA-binding proteins in the majority of membranless nuclear bodies ( Hubstenberger et al., 2017 ; Youn et al., 2019 ).…”