Sidorova J, Breeden L
Fred Hutchinson Cancer Research Center, Seattle, Washington 98104-2092.
Mol Cell Biol. 1993 Feb;13(2):1069-77. doi: 10.1128/mcb.13.2.1069-1077.1993.
SWI4 and SWI6 play a crucial role in START-specific transcription in Saccharomyces cerevisiae. SWI4 and SWI6 form a specific complex on the SCB (SWI4/6-dependent cell cycle box) sequences which have been found in the promoters of HO and G1 cyclin genes. Overproduction of SWI4 eliminates the SWI6 dependency of HO transcription in vivo and results in a new SWI6-independent, SCB-specific complex in vitro, which is heterogeneous and reacts with SWI4 antibodies. The C terminus of SWI4 is not required for SWI6-independent binding of SWI4 to SCB sequences, but it is necessary and sufficient for association with SWI6. Both SWI4 and SWI6 contain two copies of a 33-amino-acid TPLH repeat, which has been implicated in protein-protein interactions in other proteins. These repeats are not required for the SWI4-SWI6 association. Alanine substitutions in both TPLH repeats of SWI6 reduce its activity but do not affect the stability of the protein or its association with SWI4. However, these mutations reduce the ability of the SWI4/6 complex to bind DNA. Deletion of the lucine zipper motif in SWI6 also allows SWI4/6 complex formation, but it eliminates the DNA-binding ability of the SWI4/6 complex. This indicates that the integrity of two different regions of SWI6 is required for DNA binding by the SWI4/6 complex. From these data, we propose that the sequence-specific DNA-binding domain resides in SWI4 but that SWI6 controls the accessibility of this domain in the SWI4/6 complex.
SWI4和SWI6在酿酒酵母中特定起始转录过程中发挥着关键作用。SWI4和SWI6在SCB（SWI4/6依赖性细胞周期框）序列上形成特定复合物，该序列存在于HO和G1细胞周期蛋白基因的启动子中。体内过量表达SWI4可消除HO转录对SWI6的依赖性，并在体外形成一种新的不依赖SWI6的、特异性结合SCB的复合物，该复合物具有异质性且能与SWI4抗体发生反应。SWI4的C末端对于SWI4不依赖SWI6与SCB序列的结合并非必需，但对于与SWI6的结合却是必要且充分的。SWI4和SWI6都包含两个33个氨基酸的TPLH重复序列，其他蛋白质中的该重复序列与蛋白质-蛋白质相互作用有关。这些重复序列对于SWI4-SWI6的结合并非必需。SWI6的两个TPLH重复序列中的丙氨酸替代会降低其活性，但不影响蛋白质的稳定性或其与SWI4的结合。然而，这些突变会降低SWI4/6复合物结合DNA的能力。删除SWI6中的亮氨酸拉链基序也能形成SWI4/6复合物，但会消除SWI4/6复合物的DNA结合能力。这表明SWI4/6复合物结合DNA需要SWI6两个不同区域的完整性。基于这些数据，我们提出序列特异性DNA结合结构域位于SWI4中，但SWI6控制着SWI4/6复合物中该结构域的可及性。
