The mitotic spindle checkpoint plays a critical role during the cell cycle by protecting the faithful transmission of chromosomes during mitosis. If chromosomes are improperly bound to the spindle microtubules the checkpoint will prevent progress to anaphase by temporarily arresting cells in metaphase until all the chromosomes are correctly aligned. Bub1b is an essential component of the mitotic spindle checkpoint that transiently localises to kinetochores during mitosis and becomes phosphorylated, a response that is sustained during mitotic arrest. Bub1b has been implicated in other processes related to mitotic progression and is thought to regulate mitotic timing and have a role in caspase mediated cell death after prolonged mitotic arrest. The development of aneuploidy and cancer has been associated with mutations in the BUB1B gene and reduction in the level of Bub1b protein. To further our understanding of Bub1b function in the spindle checkpoint and mitosis, new protein interactions involving Bub1b were identified. This thesis describes the search for alternative proteins that interact with Bub1b, and their function in the mitotic spindle checkpoint and regulation of Bub1b activity.
Using a yeast two-hybrid approach, members of the B56 family of regulatory subunits of serine-threonine protein phosphatase (PP2A) were identified as novel interacting partners of Bub1b. Substrate specificity of PP2A is determined by the regulatory subunits. There are five characterised isoforms of the B56 family, each encoded by separate genes. In addition, some isoforms have several recognised splice variants. Confirmation of interactions by alternative methods demonstrated that the isoforms B56γ and B56[epsilon] preferentially interact with phosphorylated Bub1b, whereas the interaction of the remaining B56 isoforms (α, β and [delta]) occurs at a lower affinity with no specificity for the phosphorylated form. It was further demonstrated that B56γ1 associated with phosphorylated Bub1b in vivo.
Induced overexpression of splice variants of B56γ1 and B56γ2 demonstrated a significant reduction in levels of phosphorylated Bub1b during mitotic spindle checkpoint activation. In addition, an associated lower mitotic index was evident in cells with B56γ1 overexpression. Specific inhibition of PP2A activity with okadaic acid was shown to prolong Bub1b phosphorylation during normal mitosis and to restore the levels of phosphorylated Bub1b in arrested cells over expressing B56γ. These findings suggest a role for PP2A activity in regulation of Bub1b function that is mediated through substrate recognition by B56 regulatory subunits.
| Identifer | oai:union.ndltd.org:ADTP/217587 |
| Date | January 2006 |
| Creators | Wallis, Lise J., n/a |
| Publisher | University of Otago. Dunedin School of Medicine |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Lise J. Wallis |
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