No abstract

Highlights d Nuclear compartmentalization after mitosis requires Lem2-Nur1 and ESCRT-III/Vps4 d ESCRT-III/Vps4 remodels links between Lem2 and heterochromatin in interphase d Lem2 recruits ESCRT-III/Vps4 through Cmp7, but Vps4 pulls apart Lem2-Cmp7 complex d Lem2-Nur1 release from chromatin enables nuclear membrane sealing on the spindle

23In eukaryotes chromosomes are compartmentalized within the nucleus 24 delimited by the double membrane of the nuclear envelope (NE). Defects in the 25 function and structure of the NE are linked to disease 1,2 . During interphase, the 26 NE organizes the genome and regulates its expression 3 . As cells enter mitosis, 27 chromosomes are released from the NE, which is then remodelled to form the 28 daughter nuclei at mitotic exit 4 . Interactions between the NE and chromatin 29 underpinning both interphase and post-mitotic NE functions are executed by 30 inner nuclear membrane (INM) proteins such as members of the evolutionarily 31 conserved chromatin-binding LEM-domain family 5-8 . How chromatin tethering 32 by these transmembrane proteins is controlled in interphase and if such a 33 regulation contributes to subsequent NE dynamics in mitosis remains unclear. 34 Here we probe these fundamental questions using an emerging model 35 organism, the fission yeast Schizosaccharomyces japonicus, which breaks 36 and reforms the NE during mitosis 9,10 . We show that attachments between 37 heterochromatin and the transmembrane Lem2-Nur1 complex are 38 continuously remodelled in interphase by the ESCRT-III/AAA-ATPase Vps4 39 machinery. ESCRT-III/Vps4 mediates the release of Lem2-Nur1 from 40 heterochromatin as a prerequisite for the timely progression through mitosis. 41 Failure in this process leads to persistent association of chromosomes with 42 the INM, which prevents Lem2-Nur1 from re-localizing to the sites of NE 43 sealing around the mitotic spindle and severely delays re-establishment of 44 nucleocytoplasmic compartmentalization. Our work establishes the INM 45 transmembrane Lem2-Nur1 complex as a 'substrate' for ESCRT-III/Vps4 to 46 couple dynamic tethering of chromosomes to the INM with the establishment 47(SPB) localization 10 (Fig. 1a). Other NE proteins such as components of the nuclear 57 pore complexes (NPCs) and the second fission yeast LEM-domain protein Man1 are 58 largely excluded from the 'tails' due to their interactions with segregating 59 chromosomes at this stage of mitosis 10-12 . Thus, the 'tail' is a specialized membrane 60 domain, spatially segregated and partitioned away from the rest of the NE during 61 mitotic exit (Fig. 1a, right panel). 62 63 Lem2 orthologues function in complex with another INM protein Nur1 to promote 64 heterochromatin tethering and heterochromatic gene silencing [13][14][15] . In S. japonicus, 65Nur1 largely co-localized with Lem2 throughout the cell cycle, enriching at the NE 66 'tails' at the end of mitosis (Fig. 1b). This localization of Nur1 required Lem2, as Nur1 67 redistributed throughout the ER in lem2∆ cells (Fig. 1c). Conversely, in cells lacking 68 Nur1, less Lem2 signal was detected at the SPBs and the NE (Fig. 1d). Additionally, 69Lem2 residence time at the 'tails' was drastically reduced in Nur1-defcient cells (Fig. 70 1e). Thus, Lem2 and Nur1 appear to co-depend on each other for proper localization 71 to the NE and the SPB throughout the cell cycle. 72 73...