Cytokinesis is an essential event required to partition chromosomes, organelles and cytoplasm to daughter cells. The mechanisms that control cytokinesis are unknown, yet we know that the coordination of various signaling pathways and membrane-cytoskeleton dynamics are essential. Mammalian RACK1 (Receptor for Activated C Kinase 1) was identified in a proteomic screen of isolated CHO cell midbodies. RACK1 also co-localized with tubulin at the midbody. We subsequently showed that the C. elegans homolog, RACK-1/K04D7.1, was required during cleavage furrow completion and germline cytokinesis events (Skop et al., 2004). Structurally, RACK1 consists of seven WD40 repeats forming a seven-bladed ß-propeller, which interacts with various proteins with distinct structural folds. RACK1 has been proposed to function as an adaptor protein that anchors proteins at particular cellular locations. Being an adaptor protein, RACK1 has been shown to partner with a number of proteins such as, dynamin, G protein ßγ, PKC, PLCγ, cyclin A and
p190RhoGAP. Although several of these proteins are required during mitotic events, the role of RACK1 during the cell cycle is unknown. To characterize the role of RACK1 during mitosis, we are using spinning disk confocal microscopy combined with genetic and biochemical assays in both C. elegans and mammalian cells. In C. elegans embryos, RACK-1 localizes to centrosomes, kinetochores, cytoplasmic foci and to the midbody. Inhibition of RACK-1 expression by feeding RNAi (fRNAi) results in 45% embryonic lethality (EMB). Defects in cell cycle progression, meiotic cytokinesis, microtubule dynamics and cleavage furrow completion are observed. RACK-1 is likely required for multiple events during mitosis and possibly integrates different signaling pathways that occur prior to daughter cell separation. We are currently testing these possibilities.