Laboratory of Molecular Carcinogenesis, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA.
J Biol Chem. 2010 Nov 12;285(46):35665-74. doi: 10.1074/jbc.M110.173997. Epub 2010 Sep 9.
The mammalian SWI/SNF chromatin remodeling complex is a key player in multiple chromatin transactions. Core subunits of this complex, including the ATPase, Brg-1, and various Brg-1-associated factors (BAFs), work in concert to maintain a functional remodeling complex. This intra-complex regulation is supervised by protein-protein interactions, as stoichiometric levels of BAF proteins are maintained by proteasomal degradation. We show that the mechanism of BAF155-mediated stabilization of BAF57 involves blocking its ubiquitination by preventing interaction with TRIP12, an E3 ubiquitin ligase. Consequently, as opposed to complexed BAF57, whose principal lysines are unavailable for ubiquitination, uncomplexed BAF57 can be freely ubiquitinated and degraded by the proteasome. Additionally, a BAF57 mutant, which contains no lysine residues, was found to retain its ability to be stabilized by interaction with BAF155, suggesting that in addition to the ubiquitin-dependent mechanism of BAF57 degradation, there exists a ubiquitin-independent mechanism that may involve the direct interaction of BAF57 with the proteasome. We propose that this regulatory mechanism exists to ensure functional fidelity of the complex and prevent the accumulation of uncomplexed proteins, which may disrupt the normal activity of the complex.
哺乳动物 SWI/SNF 染色质重塑复合物是多种染色质转导中的关键因子。该复合物的核心亚基，包括 ATP 酶、Brg-1 和各种 Brg-1 相关因子（BAFs），协同工作以维持功能完整的重塑复合物。这种复合物内的调节受蛋白-蛋白相互作用的监督，因为 BAF 蛋白的化学计量水平通过蛋白酶体降解来维持。我们表明，BAF155 介导的 BAF57 稳定作用的机制涉及通过阻止与 E3 泛素连接酶 TRIP12 的相互作用来阻止 BAF57 的泛素化。因此，与复合物形式的 BAF57 不同，其主要赖氨酸不能用于泛素化，未复合的 BAF57 可以自由泛素化并被蛋白酶体降解。此外，发现一种不含赖氨酸残基的 BAF57 突变体仍能通过与 BAF155 的相互作用稳定，这表明除了 BAF57 降解的泛素依赖性机制外，还存在一种泛素非依赖性机制，可能涉及 BAF57 与蛋白酶体的直接相互作用。我们提出，这种调节机制的存在是为了确保复合物的功能保真度并防止未复合蛋白的积累，否则可能会破坏复合物的正常活性。
