Fission Yeast Dma1 And Hhp2 Are Involved In Polar Growth By Mediating The Ubiquitination And Phosphorylation Of Tip1,Respectively

Microtubules play a significant role in the establishment and maintenance of fission yeast cell polarity.The microtubule plus-end tracking protein Tip1,the CLIP 170 homologue,both affects the dynamics of microtubules and transports polarity factors to the cell ends with microtubules,thereby participating in the regulation of cell polarity.Dmal,an ubiquitin E3 ligase,is a component of the spindle assembly checkpoint(SAC)in fission yeast.Dmal is mainly localized at spindle pole body(SPB)and cell division sites,and can regulate the process of mitosis and cytokinesis.Our previous study showed that Dmal is also able to co-localize with Tip1 slightly at the cell tips.Dmal physically interacts with Tip1 and ubiquitinates Tip1 at the cell ends.In this thesis,the effect of ubiquitination of Tip1 by Dmal on cell polarity was further investigated.We found that when dmal? cells were cultured under poor nutrition condition in EMM5S and treated with HU so that the DNA replication of the cells was impaired,the frequency of bipolar growth cells was increased and the cell length was increased.Dmal also regulated the reestablishment of cell polarization during starvation exit through ubiquitination of Tip1.Our previous study revealed that deletion of the casein kinase I(CKI)homologue Hhp2 resulted in increased localization of Tip1 at cell ends.Therefore,we also launched a further study on the possible mechanism that Hhp2 regulates Tip1.We detected the interaction between Hhp2 and Tip1 and found that deletion of hhp2+significantly decreased the ubiquitination level of Tip1 and increased monopolar growth.The kinase activity of Hhp2 was required for regulation of Tip1 ubiquitination.Our results also showed that Hhp2 and Dmal regulated the ubiquitination of Tip1 and thus cell polarity through two parallel pathways.In summary,we made further studies on the mechanisms that how Tip1 is regulated for polarized growth in fission yeast,thus providing a theoretical insight into the mechanisms of polarized growth control in mammalian and human cells.