We show that the Xenopus homologs of Ndc80/Tid3/HEC1 (xNdc80) and Nuf2/MPP1/Him-10 (xNuf2) proteins physically interact in a 190-kD complex that associates with the outer kinetochore from prometaphase through anaphase. Injecting function-blocking antibodies to either xNdc80 or xNuf2 into XTC cells caused premature exit from mitosis without detectable chromosome congression or anaphase movements. Injected cells did not arrest in response to microtubule drugs, showing that the complex is required for the spindle checkpoint. Kinetochores assembled in Xenopus extracts after immunodepletion of the complex did not contain xRod, xZw10, xP150 glued (Dynactin), xMad1, xMad2, xBub1, and xBub3, demonstrating that the xNdc80 complex is required for functional kinetochore assembly. In contrast, function-blocking antibodies did not affect the localization of other kinetochore proteins when added to extracts containing previously assembled kinetochores. These extracts with intact kinetochores were deficient in checkpoint signaling, suggesting that the Ndc80 complex participates in the spindle checkpoint. We also demonstrate that the spindle checkpoint can arrest budding yeast cells lacking Ndc80 or Nuf2, whereas yeast lacking both proteins fail to arrest in mitosis. Systematic deletion of yeast kinetochore genes suggests that the Ndc80 complex has a unique role in spindle checkpoint signaling. We propose that the Ndc80 complex has conserved roles in kinetochore assembly, chromosome congression, and spindle checkpoint signaling.

Eukaryotic checkpoint genes regulate multiple cellular responses to DNA damage. In this report, we examine the roles of budding yeast genes involved in G2/M arrest and tolerance to UV exposure. A current model posits three gene classes: those encoding proteins acting on damaged DNA (e.g. RAD9 and RAD24), those transducing a signal (MEC1, RAD53 and DUN1) or those participating more directly in arrest (PDS1). Here, we define important features of the pathways subserved by those genes. MEC1, which we find is required for both establishment and maintenance of G2/M arrest, mediates this arrest through two parallel pathways. One pathway requires RAD53 and DUN1 (the 'RAD53 pathway'); the other pathway requires PDS1. Each pathway independently contributes approximately 50% to G2/M arrest, effects demonstrable after cdc13-induced damage or a double-stranded break inflicted by the HO endonuclease. Similarly, both pathways contribute independently to tolerance of UV irradiation. How the parallel pathways might interact ultimately to achieve arrest is not yet understood, but we do provide evidence that neither the RAD53 nor the PDS1 pathway appears to maintain arrest by inhibiting adaptation. Instead, we think it likely that both pathways contribute to establishing and maintaining arrest.

No abstract

All teachers hope that students learn to apply and analyze, rather than simply memorize or parrot back, the teacher's words. One method of encouraging the development of students’ higher-level thinking skills is to give learners practice in identifying appropriate analogies for biological concepts, and in forming their own. Analogies focus on the larger concepts we are trying to teach, rather than specific biological details or actual biological examples. They are fun to practice in class, and this practice prepares students for similar test questions.

No abstract