Molecular mechanisms of kinetochore capture by spindle microtubules.

For high-fidelity chromosome segregation, kinetochores must be properly captured by spindle microtubules, but the mechanisms underlying initial kinetochore capture have remained elusive. Here we visualized individual kinetochore-microtubule interactions in Saccharomyces cerevisiae by regulating the activity of a centromere. Kinetochores are captured by the side of microtubules extending from spindle poles, and are subsequently transported poleward along them. The microtubule extension from spindle poles requires microtubule plus-end-tracking proteins and the Ran GDP/GTP exchange factor. Distinct kinetochore components are used for kinetochore capture by microtubules and for ensuring subsequent sister kinetochore bi-orientation on the spindle. Kar3, a kinesin-14 family member, is one of the regulators that promote transport of captured kinetochores along microtubules. During such transport, kinetochores ensure that they do not slide off their associated microtubules by facilitating the conversion of microtubule dynamics from shrinkage to growth at the plus ends. This conversion is promoted by the transport of Stu2 from the captured kinetochores to the plus ends of microtubules.

Activation of the innate immunity in Drosophila by endogenous chromosomal DNA that escaped apoptotic degradation.

Apoptotic cell death is accompanied by degradation of chromosomal DNA. Here, we established in Drosophila a null mutation in the gene for inhibitor of caspase-activated DNase (ICAD) by P-element insertion. We also identified a loss-of-function mutant in Drosophila for DNase II-like acid DNase. The flies deficient in the ICAD gene did not express CAD, and did not undergo apoptotic DNA fragmentation during embryogenesis and oogenesis. In contrast, the deficiency of DNase II enhanced the apoptotic DNA fragmentation in the embryos and ovary, but paradoxically, the mutant flies accumulated a large amount of DNA, particularly in the ovary. This accumulation of DNA in the DNase II mutants caused the constitutive expression of the antibacterial genes for diptericin and attacin, which are usually activated during bacterial infection. The expression of these genes was further enhanced in flies lacking both dICAD and DNase II. These results indicated that CAD and DNase II work independently to degrade chromosomal DNA during apoptosis, and if the DNA is left undigested, it can activate the innate immunity in Drosophila.