Project Summary During mitosis, the mother cell divides by the formation of a cleavage furrow, leaving two daughter cells connected by a thin intercellular bridge. The resolution of this bridge, abscission, leads to the separation of the two daughter cells. During ingression of the cleavage furrow, the central spindle microtubules are compacted to form a structure known as the midbody (MB). It is now well established that MB regulates cytokinesis by recruiting abscission-mediating proteins, such as ESCRT complex, as well as several regulators of abscission checkpoint. Until recently, the MB was thought to be discarded after division by releasing it into extracellular space. However, recently it was shown that MBs accumulate in stem and cancer cells after mitosis has been completed (called MBsomes) and it has been proposed that MBsomes function as novel signalling platforms that regulate cell differentiation and proliferation. Recently we developed a protocol for purification of post- mitotic MBs and completed their proteomic and RNAseq analyses that led to identification of several mRNAs and mRNA-binding proteins that accumulate at the MB. Importantly, MB-enriched mRNAs encode several ESCRT complex subunits, as well as proteins that stimulate cell proliferation. Furthermore, we show that these MB-enriched mRNAs can be transferred to the neighboring cells via post-mitotic MB internalization. Based on all of these findings, we hypothesize that targeting of selected mRNAs to the MB during cytokinesis play a key role in regulating cell abcission and post-mitotic MBsome signaling. Here we propose three specific aims to test this hypothesis. First, we will map and characterize the domain(s) within 3'-UTR that are needed for mRNA targeting during cytokinesis. We will then use candidate approach, as well as proteomic and CRISPR screens, to identify RNA-binding proteins that interact with these 3”-UTR domains and regulate mRNA targeting and localized translation at the MB. Second, we will test the possibility that MB accumulation/translation of ESCRT mRNAs mediates ESCRP complex targeting to the MB. Third, we will test whether MBsome-dependent transfer of specific mRNAs, such as mRNA encoding proliferation regulator CENP-E contribute to MBsome-induced cell proliferation.