MAN1 and emerin have overlapping function(s) essential for chromosome segregation and cell division in Caenorhabditis elegans

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Abstract
Emerin and MAN1 are LEM domain-containing integral membrane proteins of the vertebrate nuclear envelope. The function of MAN1 is unknown, whereas emerin is known to interact with nuclear lamins, barrier-to-autointegration factor (BAF), nesprin-1α, and a transcription repressor. Mutations in emerin cause X-linked recessive Emery–Dreifuss muscular dystrophy. Emerin and MAN1 homologs are both conserved in Caenorhabditis elegans , but loss of Ce-emerin has no detectable phenotype. We therefore used C. elegans to test the hypothesis that Ce-MAN1 overlaps functionally with Ce-emerin. Supporting this model, Ce-MAN1 interacted directly with Ce-lamin and Ce-BAF in vitro and required Ce-lamin for its nuclear envelope localization. Interestingly, RNA interference-mediated removal of ≈90% of Ce-MAN1 was lethal to ≈15% of embryos. However, in the absence of Ce-emerin, ≈90% reduction of Ce-MAN1 was lethal to all embryos by the 100-cell stage, with a phenotype involving repeated cycles of anaphase chromosome bridging and cytokinesis [“cell untimely torn” ( cut ) phenotype]. Immunostaining showed that the anaphase-bridged chromatin specifically retained a mitosis-specific phosphohistone H3 epitope and failed to recruit detectable Ce-lamin or Ce-BAF. These findings show that LEM domain proteins are essential for cell division and that Ce-emerin and Ce-MAN1 share at least one and possibly multiple overlapping functions, which may be relevant to Emery–Dreifuss muscular dystrophy.