A heterotetrameric protein complex playing a key role in the formation of the central spindle in mitosis. Made up of two molecules each of a mitotic kinesin (ZEN-4 in Caenorhabditis elegans or MKLP1 in mammals) and of two molecules each of a GTPase activating protein (GAP) factor (CYK-4 in Caenorhabditis elegans or MgcRacGAP in mammals).