RESUMEN
The microtubule-associated protein, TPX2, regulates the activity of the mitotic kinesin, Eg5, but the mechanism of regulation is not established. Using total internal reflection fluorescence microscopy, we observed that Eg5, in extracts of mammalian cells expressing Eg5-EGFP, moved processively toward the microtubule plus-end at an average velocity of 14 nm/s. TPX2 bound to microtubules with an apparent dissociation constant of â¼ 200 nm, and microtubule binding was not dependent on the C-terminal tails of tubulin. Using single molecule assays, we found that full-length TPX2 dramatically reduced Eg5 velocity, whereas truncated TPX2, which lacks the domain that is required for the interaction with Eg5, was a less effective inhibitor at the same concentration. To determine the region(s) of Eg5 that is required for interaction with TPX2, we performed microtubule gliding assays. Dimeric, but not monomeric, Eg5 was differentially inhibited by full-length and truncated TPX2, demonstrating that dimerization or residues in the neck region are important for the interaction of TPX2 with Eg5. These results show that both microtubule binding and interaction with Eg5 contribute to motor inhibition by TPX2 and demonstrate the utility of mammalian cell extracts for biophysical assays.
Asunto(s)
Cinesinas/metabolismo , Microtúbulos/metabolismo , Animales , Proteínas de Ciclo Celular/química , Proteínas de Ciclo Celular/genética , Proteínas de Ciclo Celular/metabolismo , Proteínas Fluorescentes Verdes/genética , Proteínas Fluorescentes Verdes/metabolismo , Humanos , Cinesinas/química , Cinesinas/genética , Células LLC-PK1 , Microscopía Fluorescente , Proteínas Asociadas a Microtúbulos/química , Proteínas Asociadas a Microtúbulos/genética , Proteínas Asociadas a Microtúbulos/metabolismo , Modelos Biológicos , Proteínas Motoras Moleculares/química , Proteínas Motoras Moleculares/genética , Proteínas Motoras Moleculares/metabolismo , Proteínas Nucleares/química , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Fragmentos de Péptidos/química , Fragmentos de Péptidos/genética , Fragmentos de Péptidos/metabolismo , Unión Proteica , Dominios y Motivos de Interacción de Proteínas , Estructura Cuaternaria de Proteína , Proteínas Recombinantes de Fusión/química , Proteínas Recombinantes de Fusión/genética , Proteínas Recombinantes de Fusión/metabolismo , PorcinosRESUMEN
Kinesin-5 is an essential mitotic motor. However, how its spatial-temporal distribution is regulated in mitosis remains poorly understood. We expressed localization and affinity purification-tagged Eg5 from a mouse bacterial artificial chromosome (this construct was called mEg5) and found its distribution to be tightly regulated throughout mitosis. Fluorescence recovery after photobleaching analysis showed rapid Eg5 turnover throughout mitosis, which cannot be accounted for by microtubule turnover. Total internal reflection fluorescence microscopy and high-resolution, single-particle tracking revealed that mEg5 punctae on both astral and midzone microtubules rapidly bind and unbind. mEg5 punctae on midzone microtubules moved transiently both toward and away from spindle poles. In contrast, mEg5 punctae on astral microtubules moved transiently toward microtubule minus ends during early mitosis but switched to plus end-directed motion during anaphase. These observations explain the poleward accumulation of Eg5 in early mitosis and its redistribution in anaphase. Inhibition of dynein blocked mEg5 movement on astral microtubules, whereas depletion of the Eg5-binding protein TPX2 resulted in plus end-directed mEg5 movement. However, motion of Eg5 on midzone microtubules was not altered. Our results reveal differential and precise spatial and temporal regulation of Eg5 in the spindle mediated by dynein and TPX2.
Asunto(s)
Proteínas de Ciclo Celular/metabolismo , Dineínas/metabolismo , Cinesinas/metabolismo , Proteínas Asociadas a Microtúbulos/metabolismo , Mitosis/fisiología , Proteínas Nucleares/metabolismo , Huso Acromático/metabolismo , Animales , Secuencia de Bases , Proteínas de Ciclo Celular/antagonistas & inhibidores , Proteínas de Ciclo Celular/genética , Recuperación de Fluorescencia tras Fotoblanqueo , Cinesinas/genética , Células LLC-PK1 , Ratones , Microscopía Fluorescente , Proteínas Asociadas a Microtúbulos/antagonistas & inhibidores , Proteínas Asociadas a Microtúbulos/genética , Microtúbulos/metabolismo , Proteínas Nucleares/antagonistas & inhibidores , Proteínas Nucleares/genética , ARN Interferente Pequeño/genética , Proteínas Recombinantes de Fusión/genética , Proteínas Recombinantes de Fusión/metabolismo , PorcinosRESUMEN
Mitotic spindle assembly requires the regulated activity of numerous spindle-associated proteins. In mammalian cells, the Kinesin-5 motor Eg5 interacts with the spindle assembly factor TPX2, but how this interaction contributes to spindle formation and function is not established. Using bacterial artificial chromosome technology, we generated cells expressing TPX2 lacking the Eg5 interaction domain. Spindles in these cells were highly disorganized with multiple spindle poles. The TPX2-Eg5 interaction was required for kinetochore fiber formation and contributed to Eg5 localization to spindle microtubules but not spindle poles. Microinjection of the Eg5-binding domain of TPX2 resulted in spindle elongation, indicating that the interaction of Eg5 with TPX2 reduces motor activity. Consistent with this possibility, we found that TPX2 reduced the velocity of Eg5-dependent microtubule gliding, inhibited microtubule sliding, and resulted in the accumulation of motor on microtubules. These results establish a novel function of TPX2 in regulating the location and activity of the mitotic motor Eg5.