Department of Biochemistry and Molecular Biology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205, USA.
Mol Biol Cell. 2010 May 1;21(9):1609-19. doi: 10.1091/mbc.e09-11-0923. Epub 2010 Mar 10.
The yeast SBF transcription factor is a heterodimer comprised of Swi4 and Swi6 that has a well defined role in cell cycle-specific transcription. SBF serves a second function in the transcriptional response to cell wall stress in which activated Mpk1 mitogen-activated protein kinase of the cell wall integrity signaling pathway forms a complex with Swi4, the DNA binding subunit of SBF, conferring upon Swi4 the ability to bind DNA and activate transcription of FKS2. Although Mpk1-Swi4 complex formation and transcriptional activation of FKS2 does not require Mpk1 catalytic activity, Swi6 is phosphorylated by Mpk1 and must be present in the Mpk1-Swi4 complex for transcriptional activation of FKS2. Here, we find that Mpk1 regulates Swi6 nucleocytoplasmic shuttling in a biphasic manner. First, formation of the Mpk1-Swi4 complex recruits Swi6 to the nucleus for transcriptional activation. Second, Mpk1 negatively regulates Swi6 by phosphorylation on Ser238, which inhibits nuclear entry. Ser238 neighbors a nuclear localization signal (NLS) whose function is blocked by phosphorylation at Ser238 in a manner similar to the regulation by Cdc28 of another Swi6 NLS, revealing a mechanism for the integration of multiple signals to a single endpoint. Finally, the Kap120 beta-importin binds the Mpk1-regulated Swi6 NLS but not the Cdc28-regulated NLS.
酵母 SBF 转录因子是由 Swi4 和 Swi6 组成的异二聚体，在细胞周期特异性转录中具有明确的作用。SBF 在细胞应对细胞壁应激的转录反应中具有第二个功能，其中细胞壁完整性信号通路中激活的 Mpk1 丝裂原激活蛋白激酶与 SBF 的 DNA 结合亚基 Swi4 形成复合物，赋予 Swi4 结合 DNA 的能力，并激活 FKS2 的转录。尽管 Mpk1-Swi4 复合物的形成和 FKS2 的转录激活不需要 Mpk1 的催化活性，但 Swi6 被 Mpk1 磷酸化，并且必须存在于 Mpk1-Swi4 复合物中才能激活 FKS2 的转录。在这里，我们发现 Mpk1 以双相方式调节 Swi6 的核质穿梭。首先，Mpk1-Swi4 复合物的形成将 Swi6 募集到细胞核中以进行转录激活。其次，Mpk1 通过 Ser238 上的磷酸化负调控 Swi6，从而抑制核进入。Ser238 紧邻核定位信号（NLS），其功能被 Ser238 磷酸化阻断，类似于 Cdc28 对另一个 Swi6 NLS 的调节，揭示了一种将多个信号整合到单个终点的机制。最后，Kap120 β-importin 结合 Mpk1 调节的 Swi6 NLS，但不结合 Cdc28 调节的 NLS。
