Characterizing the function of Int6 in fission yeast.

摘要

Mutations in mammalian INT6 genes have been implicated in breast cancer formation, but very little is known about the function of Int6. Our laboratory has identified an Int6 homolog from fission yeast, Yin6, by its binding to a conserved protein in the Ras G-protein pathway, Moe1. The aim of this study is to exploit the power of yeast genetics to better understand the conserved functions of Int6 that are important for tumorigenesis in humans.; My data reveal that Yin6 and Moe1 bind directly and they require each other for proper levels and localizations. The Yin6-Moe1 complex cooperates with Ras to maintain the fidelity of chromosome transmission. Deletion of yin6 (yin6Δ) weakly impairs chromosome disjunction, but yin6Δ together with rasΔ causes severe chromosome missegregation. One of the key mechanisms through which Yin6 and Ras regulate chromosome segregation is by mediating the function of the proteasome, a multisubunit protease that is essential in diverse cellular processes including chromosome separation. Yin6 physically associates with the proteasome and Yin6 inactivation weakens proteasome activities. In yin6Δ cells, protein degradation occurs less efficiently and securin, a known proteasome substrate involved in chromosome disjunction, accumulates abnormally. Yin6 controls proteasome function by preferentially binding to and affecting the localization/assembly of a proteasome component, Rpn5. Rpn5 is required for the proper integrity of the proteasome. Normal Ras and Moel function is also crucial for proper proteasome function and Rpn5 localization. Human Int6 (h-Int6) binds h-Moe1 and h-Rpn5. Wild-type h-Int6, but not mutant forms of h-Int6 found in tumors, rescues all the phenotype associated with yin6Δ and can substitute Yin6 to restore Rpn5 localization in yeast, suggesting that mediating chromosome segregation via the proteasome is a conserved function of Int6 proteins.; Collectively, these data provide novel insights into the function of Int6 and Ras in mediating chromosome segregation and proteasome activities. Moreover, they support a model whereby INT6 mutations in humans either alone or together with additional mutations, such as RAS mutation, might contribute to tumorigenesis by inhibiting proteasome functions, which leads to inefficient degradation of mitotic regulators, thus affecting cell division and chromosome stability.