TPX2, a novel Xenopus MAP involved in spindle pole organization

TPX2, the targeting protein for Xenopus kinesin-like protein 2 (Xklp2), was identified as a microtubule-associated protein that mediates the binding o f the COOH-terminal domain of Xklp2 to microtubules (Wittmann, T., H. Bolet i, C. Antony, E. Karsenti, and I. Vernos. 1998. J. Cell Biol. 143:673-685). Here, we re port the cloning and functional characterization of Xenopus TP X2. TPX2 is a novel, basic 82.4-kD protein that is phosphorylated during mi tosis in a microtubule-dependent way. TPX2 is nuclear during interphase and becomes localized to spindle poles in mitosis. Spindle pole localization o f TPX2 requires the activity of the dynein-dynactin complex. In late anapha se TPX2 becomes relocalized from the spindle poles to the midbody. TPX2 is highly homologous to a human protein of unknown function and thus defines a new family of vertebrate spindle pole components. We investigated the func tion of TPX2 using spindle assembly in Xenopus egg extracts. Immunodepletio n of TPX2 from mitotic egg extracts resulted in bipolar structures with dis integrating poles and a decreased microtubule density. Addition of an exces s of TPX2 to spindle assembly reactions gave rise to monopolar structures w ith abnormally enlarged poles. We conclude that, in addition to its functio n in targeting Xklp2 to microtubule minus ends during mitosis, TPX2 also pa rticipates in the organization of spindle poles.