We have identified a unique human microtubule-associated protein (MAP) named ASAP for ASter-Associated Protein. ASAP localizes to microtubules in interphase, associates with the mitotic spindle during mitosis, localizes to the central body during cytokinesis and directly binds to purified microtubules by its COOHterminal domain. Overexpression of ASAP induces profound bundling of cytoplasmic microtubules in interphase cells and aberrant monopolar spindles in mitosis. Depletion of ASAP by RNA interference results in severe mitotic defects: it provokes aberrant mitotic spindle, delays mitotic progression, and leads to defective cytokinesis or cell death. These results suggest a crucial role for ASAP in the organization of the bipolar mitotic spindle, mitosis progression, and cytokinesis and define ASAP as a key factor for proper spindle assembly. mitosis D ivision of the cell requires coordination between chromosome segregation by a bipolar microtubule (MT)-based structure, the mitotic spindle, and cleavage of the cell by the cytokinetic apparatus. The transition from interphase to mitosis or meiosis is accompanied by a dramatic reorganization of the MT cytoskeleton. The assembly of bipolar spindles is preceded by the movement of centrosomes toward opposite poles of the cell, where these structures function as nucleation sites for MT polymerization (1-3). Spindle assembly and chromosome segregation require MT motor proteins including kinesin-like proteins and cytoplasmic dynein, nonmotor MT-associated proteins (MAPs) required for the regulation of MT dynamics and other molecules, including the small GTPase Ran involved in MT polymerization during mitosis (2, 3). Proper assembly and function of the bipolar spindle is essential for genomic stability (1), and identification of key factors involved in this process as well as knowledge of their regulation are crucial.To ensure high-fidelity DNA transmission during cell division, both chromosome segregation and cytokinesis must be tightly coordinated. Cytokinesis is accomplished by the contraction of an acto-myosin ring that leads to daughter cell separation at the midbody (4). A number of studies have suggested that the temporal and spatial organization of the cytokinetic machine is under the control of the mitotic spindle (5). The spindle midzone composed of highly bundled MTs, originating from the opposite poles, plays an essential role in the initiation and completion of cell cleavage, and is the binding site for a number of proteins that play a part in cytokinesis (6). Among these proteins are passenger proteins (7-9), protein kinases (10), MT motor proteins (11, 12), and MAPs (13). However, the mechanism of central spindle assembly, the molecular basis for maintenance of the midzone MT bundle and the functional roles of these midzone-associated proteins are still largely unknown. Thus, identification of previously undescribed factors involved in both mitosis and cytokinesis may shed light on these different mechanisms.In this paper, we report the identification of...

This study shows that capping protein (CP) is essential for mammalian cell migration in vitro and in vivo. The authors also show that CP is present in filopodia of multiple cell types and that it regulates filopodial structure and function. Thus CP function in both lamellipodia and filopodia may contribute to efficient migration.