Role for the Mortality Factors MORF4, MRGX, and MRG15 in Transcriptional Repression via Associations with Pf1, mSin3A, and Transducin-Like Enhancer of Split

mSin3A and Transducin-Like Enhancer of Split (TLE) are two histone deacetylase (HDAC)-containing corepressors that function to repress transcription at targeted genes. Pf1 is a plant homeodomain zinc finger protein that interacts with both mSin3A and TLE, suggesting that it coordinates their function. Here we show that mSin3A and TLE interact with members of the mortality factor (MORF) family of putative transcriptional regulators. This family comprises MORF on chromosome 4 (MORF4) and MORF-related genes on chromosomes X and 15 (MRGX and MRG15, respectively) and is proposed to contribute to cellular senescence. Consistent with a role in transcription, we demonstrate that Gal4 fusions to each MORF family member repress transcription from a Gal4-dependent luciferase reporter. By using both mapping experiments and a dominant negative form of TLE, we show that repression by MORFs requires associations with mSin3A and TLE. Therefore, common functions of the MORFs are likely elicited through the action of a MORF/mSin3A/TLE complex. While the MORFs may have common functions, MRG15, but not MRGX or MORF4, interacted with Pf1. Therefore, MRG15 may have functions that are distinct from those of MRGX and MORF4. Consistent with this hypothesis, Pf1 reduced transcriptional repression by Gal4-MRG15 but it had no effect on repression by MRGX and MORF4. Pf1 has independent binding sites for MRG15 and mSin3A. In addition, Pf1 and MRG15 bind different domains on mSin3A. Together, these data suggest that the unique functions of MRG15 are elicited through the action of an MRG15/Pf1/mSin3A complex.

mSin3A与Split类转导增强子（Transducin-Like Enhancer of Split，TLE）是两类携带组蛋白去乙酰化酶（histone deacetylase，HDAC）的共阻遏因子，可在靶基因位点发挥转录阻遏功能。Pf1是一种植物同源结构域锌指蛋白（plant homeodomain zinc finger protein），能够与mSin3A和TLE结合，提示其可协同二者的功能。本研究证实，mSin3A与TLE可与推定转录调节因子——死亡率因子（mortality factor，MORF）家族成员发生相互作用。该家族包含4号染色体上的MORF（MORF4）以及X、15号染色体上的MORF相关基因（分别为MRGX和MRG15），有研究提出该家族参与细胞衰老过程。与转录调控功能相符，我们发现，与Gal4融合的各MORF家族成员均可抑制Gal4依赖型荧光素酶报告基因的转录。通过定位实验与显性负效TLE突变体，我们证实MORF家族的转录阻遏作用依赖于与mSin3A和TLE的结合。因此，MORF家族的共同功能可能通过MORF/mSin3A/TLE复合物介导实现。尽管MORF家族成员具备共性功能，但MRG15可与Pf1结合，而MRGX与MORF4则无此相互作用。由此提示，MRG15可能具备与MRGX和MORF4不同的独特功能。与该假说一致，Pf1可削弱Gal4-MRG15介导的转录阻遏，但对MRGX与MORF4的阻遏活性无影响。Pf1拥有独立的MRG15与mSin3A结合位点。此外，Pf1与MRG15在mSin3A上结合不同的结构域。综上，上述实验数据表明，MRG15的独特功能可能通过MRG15/Pf1/mSin3A复合物介导实现。