Rho1 and Rgf3 regulate the expansion of the nuclear envelope during fission yeast mitosis/cytokinesis

The nuclear envelope (NE) surrounds the genetic material and is continuous with the endoplasmic reticulum (ER). In yeast and other organisms undergoing closed mitosis, nuclear envelope expansion (NME) is strictly required to accommodate spindle elongation and ensure proper chromosome segregation within a single nuclear compartment. Failure to expand the NE during mitosis leads to chromosome missegregation. Here, we show that deletion of the unstructured N-terminal domain of Rgf3, a Rho1-specific guanine nucleotide exchange factor (GEF), causes early mitotic defects that produce the characteristic "cut" phenotype of untimely cell division. The rgf3{Delta}N2 mutant displays spindle buckling, a hallmark of anaphase nuclei unable to properly expand the NE. From yeast to mammals, phosphatidic acid (PA)--a key precursor in phospholipid biosynthesis--is metabolized via two competing pathways, the cytidine diphosphate-diacylglycerol (CDP-DAG) and the Kennedy pathways, both contributing to lipid membrane homeostasis. We provide evidence that impaired Rho1 activation in rgf3{Delta}N2 selectively disrupts phospholipid synthesis through the CDP-choline branch of the Kennedy pathway. Thus, Rho1 promotes mitotic progression by modulating phospholipid biosynthesis to enable efficient NME during anaphase. HighlightsThe N-terminus of Rgf3 is required for proper nuclear envelope expansion (NME) during anaphase. The structurally flexible N-terminal domain of Rgf3 is essential for localized Rho1 activation. Active Rho1 drives mitotic membrane growth by modulating phospholipid synthesis through the Kennedy pathway.