Lyons D M, Mahanty S K, Choi K Y, Manandhar M, Elion E A
Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA.
Mol Cell Biol. 1996 Aug;16(8):4095-106. doi: 10.1128/MCB.16.8.4095.
The mating mitogen-activated protein kinase (MAPK) cascade has three major outputs prior to fusion: transcriptional activation of many genes, cell cycle arrest in the G1 phase, and polarized growth. Bem1 localizes near the cortical actin cytoskeleton and is essential for polarized growth during mating. Here we show that Bem1 is required for efficient signal transduction and coordinates MAPK cascade activation with G1 arrest and mating. bem1delta null mutants are defective in G1 arrest and transcriptional activation in response to mating pheromone. Bem1 protein stimulates Fus3 (MAPK) activity and associates with Ste5, the tethering protein essential for activation of the MAPK kinase kinase Ste11. Bem1-Ste5 complexes also contain Ste11, Ste7 (MAPK kinase), and Fus3, suggesting that Ste5 localizes the MAPK cascade to Bem1. Strikingly, Bem1 also copurifies with Far1, a Fus3 substrate required for G1 arrest and proper polarized growth during mating. These and other results suggest that Bem1 may cross-link the Ste5-MAPK cascade complex to upstream activators and specific downstream substrates at the shmoo tip, thus enabling efficient circuitry for G1 arrest and mating.
在融合之前，交配型丝裂原活化蛋白激酶（MAPK）级联有三个主要输出：许多基因的转录激活、G1期细胞周期停滞以及极性生长。Bem1定位于皮质肌动蛋白细胞骨架附近，是交配过程中极性生长所必需的。在此我们表明，Bem1是有效信号转导所必需的，并且能将MAPK级联激活与G1停滞和交配进行协调。bem1δ缺失突变体在响应交配信息素时的G1停滞和转录激活方面存在缺陷。Bem1蛋白刺激Fus3（MAPK）活性，并与Ste5相关联，Ste5是激活MAPK激酶激酶Ste11所必需的锚定蛋白。Bem1 - Ste5复合物还包含Ste11、Ste7（MAPK激酶）和Fus3，这表明Ste5将MAPK级联定位于Bem1。引人注目的是，Bem1还与Far1共同纯化，Far1是交配过程中G1停滞和正确极性生长所需的Fus3底物。这些以及其他结果表明，Bem1可能在芽管尖端将Ste5 - MAPK级联复合物与上游激活剂和特定下游底物交联起来，从而实现用于G1停滞和交配的高效信号通路。
