Nascent lipid droplet (LD) formation occurs in the endoplasmic reticulum (ER) membrane but it is not known how sites of biogenesis are determined. We previously identified ER domains in S. cerevisiae containing the reticulon homology domain (RHD) protein Pex30 that are regions where preperoxisomal vesicles (PPVs) form. Here, we show that Pex30 domains are also sites where most nascent LDs form. Mature LDs usually remain associated with Pex30 subdomains, and the same Pex30 subdomain can simultaneously associate with a LD and a PPV or peroxisome. We find that in higher eukaryotes multiple C2 domain containing transmembrane protein (MCTP2) is similar to Pex30: it contains an RHD and resides in ER domains where most nascent LD biogenesis occurs and that often associate with peroxisomes. Together, these findings indicate that most LDs and PPVs form and remain associated with conserved ER subdomains, and suggest a link between LD and peroxisome biogenesis.

While cytokinesis has been intensely studied, the way it is executed during development is not well understood, despite a long-standing appreciation that various aspects of cytokinesis vary across cell and tissue types. To address this, we investigated cytokinesis during the invariant C. elegans embryonic divisions and found several reproducibly altered parameters at different stages. During early divisions, furrow ingression asymmetry and midbody inheritance is consistent, suggesting specific regulation of these events. During morphogenesis, we found several unexpected alterations to cytokinesis including apical midbody migration in polarizing epithelial cells of the gut, pharynx and sensory neurons. Aurora B kinase, which is essential for several aspects of cytokinesis, remains apically localized in each of these tissues after internalization of midbody ring components. Aurora B inactivation disrupts cytokinesis and causes defects in apical structures, even if inactivated post-mitotically. Therefore, cytokinesis is implemented in a specialized way during epithelial polarization and Aurora B has a new role in the formation of the apical surface.

Microhomology-mediated end-joining (MMEJ) is an intrinsically mutagenic pathway of DNA double strand break repair essential for proliferation of homologous recombination (HR) deficient tumors. While targeting MMEJ has emerged as a powerful strategy to eliminate HR-deficient (HRD) cancers, this is limited by an incomplete understanding of the mechanism and factors required for MMEJ repair. Here, we identify the APE2 nuclease as a novel MMEJ effector. We show that loss of APE2 blocks the fusion of deprotected telomeres by MMEJ and inhibits MMEJ in DNA repair reporter assays to levels comparable to Pol Theta suppression. Mechanistically, we demonstrate that APE2 possesses intrinsic flap-cleaving activity, that its MMEJ function in cells depends on its nuclease domain and further identify uncharacterized domains required for recruitment to damaged DNA. We conclude that HR-deficient cells are addicted to APE2 due to a previously unappreciated role in MMEJ, which could be exploited in the treatment of cancer.

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29While cytokinesis has been intensely studied, how it is executed during 30 development is not well understood, despite a long-standing appreciation that various 31 aspects of cytokinesis vary across cell and tissue types. To address this, we 32 investigated cytokinesis during the invariant C. elegans embryo lineage and found 33 several reproducibly altered parameters at different stages. During early divisions, 34 furrow ingression asymmetry and midbody inheritance is consistent, suggesting 35 specific regulation of these events. During morphogenesis, we find several 36 unexpected alterations including migration of midbodies to the apical surface during 37 epithelial polarization in different tissues. Aurora B kinase, which is essential for 38