Balanced RCC1 activity organizes the specialized spindle midplane during cleavage divisions

Chromosome-bound RCC1 generates Ran-GTP signals to organize functional spindles for faithful chromosome segregation during mitosis and meiosis. RCC1 is the sole guanine nucleotide exchange factor (GEF) for Ran and is essential for spindle assembly during early, but not late, embryonic divisions. However, how RCC1 organizes the specialized embryonic spindle and when its function changes during early embryogenesis remain unclear. Here, using time-resolved RCC1 depletion and depletion-rescue experiments in medaka embryos, we show that RCC1 GEF activity is specifically required before the blastula stage to organize a specialized metaphase spindle mid-plane that ensures faithful chromosome segregation. Mechanistically, RCC1 promotes the accumulation of the canonical Ran effectors HURP and KIFC1/HSET, and unexpectedly, the microtubule motor dynein at the spindle midplane during early embryonic divisions. Intriguingly, a five-fold increase in RCC1 expression phenocopies RCC1 depletion, disrupting spindle-midplane organization and the accumulation of KIFC1 and dynein in a GEF activity-dependent manner. Together, our findings demonstrate that both insufficient and excessive RCC1 GEF activity compromise embryonic spindle assembly, revealing that balanced Ran activation is required to organize the specialized spindle midplane during vertebrate cleavage divisions.