Centrosomes are the major microtubule-nucleating organelles in most vertebrate cells (Doxsey, 2001b
). In mitosis, they contribute to spindle organization and function, and in interphase, they organize microtubule arrays that serve as tracks for transporting proteins, organelles, and chromosomes. The centrosome also anchors regulatory molecules and may serve as a central site that receives, integrates, and transmits signals that regulate fundamental cellular functions. The core of the centrosome is comprised of a pair of centrioles, microtubule barrels that appear to anchor microtubules (Chretien et al., 1997
; Piel et al., 2000
). Each centriole is surrounded by pericentriolar material or centrosome matrix, which nucleates the growth of new microtubules and seems to be organized by the centrioles (Bobinnec et al., 1998
). Although best known for their role in microtubule nucleation, recent data suggest that centrosomes also play key roles in cytokinesis and cell cycle progression.
A role for centrosomes in defining the site of cell cleavage during cytokinesis has been suggested for some time (Rappaport, 1986
). Recent studies with vertebrate cells provide evidence for a direct link between centrosome activity and completion of cytokinesis. Elimination of centrosomes from interphase cells by removal with a microneedle (Hinchcliffe et al., 2001
) or from mitotic cells by laser ablation (Khodjakov and Rieder, 2001
) caused cytokinesis defects, arrest, or failure. In another study, it was shown that during the final stages of cytokinesis, the maternal centriole moved to the intercellular bridge, the microtubule-filled interconnection between nascent daughter cells (Piel et al., 2001
). Centriole repositioning correlated with bridge narrowing and microtubule depolymerization, while movement of the centriole away from the bridge correlated with cell cleavage or abscission. The authors suggested that the maternal centriole might anchor a regulatory pathway that controls the final stages of cell division in vertebrate cells. This would be analogous to regulatory pathways anchored at spindle pole bodies (the centrosome equivalent) in budding and fission yeasts that control mitotic exit and cytokinesis (for reviews see Bardin and Amon, 2001
; McCollum and Gould, 2001
; Pereira and Schiebel, 2001
). However, no vertebrate pathway analogous to the mitotic exit network (MEN)*
in budding yeast or septation initiation network (SIN) in fission yeast has been identified (Glotzer, 2001
; Guertin et al., 2002
). Moreover, the role of centrosome-associated molecules in the process of cytokinesis is poorly understood.
In addition to their role in cytokinesis, centrosomes appear to have a role in cell cycle progression. Recent evidence demonstrates that vertebrate cells lacking centrosomes do not initiate DNA replication (Hinchcliffe et al., 2001
; Khodjakov and Rieder, 2001
). The authors suggested that centrosomes controlled entry into S phase by recruiting or concentrating “core” centrosome molecules required for this process or that they indirectly activated a cellular checkpoint that monitored aberrant centrosome number. In another experimental system, vertebrate cells treated with cytochalasin D to inhibit actin-mediated cell cleavage also arrested cells in G1 as binucleate cells with supernumerary centrosomes (Andreassen et al., 2001
). Although these results suggest that changes in centrosome number can affect entry into S phase, the precise role of centrosomes in cell cycle progression in vertebrate cells will require identification of the molecular components and pathways that control these events.
In this paper, we identify a novel component of the vertebrate maternal centriole called centriolin. Abrogation of centriolin function by small interfering RNA (siRNA) silencing, overexpression, or antibody inhibition produces cytokinesis failure and G1/G0 arrest, just as seen when centrosomes are experimentally eliminated from cells. Centriolin silencing produces a novel cytokinesis phenotype in which dividing cells remain interconnected by long strands of cytoplasm and fail to cleave. The cytokinesis activity lies in a centriolin domain that is homologous to the MEN/SIN components Nud1p/Cdc11p and binds the Nud1p-interacting GTPase-activating protein Bub2p. We conclude that centriolin is required for a distinct step in the final stages of vertebrate cytokinesis and can influence entry into S phase.