SRBD1, a highly conserved gene required for chromosome individualization

Despite significant progress made in functional genomics, the roles of many genes remain enigmatic. Nevertheless, the function of most genes that are essential for cell viability, such as those required for ribosome biogenesis, mRNA processing or cell division have been determined to date. Indeed, analysis of the genes that are essential across human cell lines indicated that their functions are largely resolved. Here, we study an exception, the S1 domain containing protein 1 (SRBD1); an absolutely essential gene lacking any assigned function. Through genetic, proteomic and functional approaches, we discovered that SRBD1 is a DNA binding protein and is a key component of the mitotic chromatid axis. Loss of SRBD1 results in a severe defect in sister chromatid segregation due to a failure in their individualization in prophase. Finally, using genetic screens, we uncovered that the essentiality of SRBD1 strictly requires the presence of condensin II, but not condensin I.