| Popis: |
Spontaneous mutations in the human LIS1 gene are responsible for Type I lissencephaly ("smooth brain"). The distribution of neurons within the cerebral cortex of lissencephalic children appears randomized, probably owing to a defect in neuronal migration during early development. LIS1 has been implicated in the dynein pathway by genetic analyses in fungi. We previously reported that the vertebrate LIS1 co-localized with dynein at prometaphase kinetochores, and interference with LIS1 function at kinetochore caused misalignment of chromosomes onto the metaphase plate. This leads to a hypothesis that LIS1 might regulate kinetochore protein targeting. In order to test this hypothesis, I created dominant inhibitory constructs of LIS1. After removal of the endogenous LIS1 from the kinetochore by overexpression of the N-terminal self-association domain of LIS1, dynein and dynactin remained at the kinetochores. This result indicated that LIS1 is not required for dynein to localize at the kinetochore. Next, CLIP-170 was displaced from the kinetochores in the LIS1 full-length and the C-terminal WD-repeat overexpressers, suggesting a role for LIS1 in targeting CLIP-170 onto kinetochores. LIS1 was co-immunoprecipitated with dynein and dynactin. Its association with kinetochores was mediated by dynein and dynactin, suggesting LIS1 might interact directly with subunits of dynein and/or dynactin complexes. I found that LIS1 interacted with the heavy and intermediate chains (HC and IC) of dynein complex, and the dynamitin subunit of dynactin complex. In addition to kinetochore targeting, the LIS1 C-terminal WD-repeat domain was responsible for interactions with dynein and dynactin. Interestingly, LIS 1 interacted with two distinct sites on HC: one in the stem region containing the subunit-binding domain, and the other in the first AAA motif of the motor domain, which is indispensable for the ATPase function of the motor protein. This LIS1-dynein motor domain interaction suggests a role for LIS1 in regulating dynein motor activity. To test this hypothesis, changes of dynein ATPase activity was measured in the presence of LIS1 protein. The ATPase activity of dynein was stimulated by the addition of a recombinant LIS1 protein. Besides kinetochores, others and we have found LIS1 also localized at microtubule plus ends. LIS1 may mediate dynein and dynactin mitotic functions at these ends by interacting with astral microtubules at cortex, and associating with the spindle microtubules at kinetochores. Overexpression of LIS1 displaced dynein and dynactin from the microtubule plus ends, and mitotic progression was severely perturbed in LIS1 overexpressers. These results suggested that the role for LIS1 at microtubule plus ends is to regulate dynein and dynactin interactions with various subcellular structures. Results from my thesis research clearly favored the conclusion that LIS1 activates dynein ATPase activity through its interaction with the motor domain, and this activation is important to establish an interaction between dynein and microtubule plus ends during mitosis. I believe that my thesis work not only has provided ample implications regarding dynein dysfunction in disease formation, but also has laid a significant groundwork for more future studies in regulations of the increasing array of dynein functions. |
