The C-terminal SET domains of ALL-1 and TRITHORAX interact with the INI1 and SNR1 proteins, components of the SWI/SNF complex

The ALL-1 gene was discovered by virtue of its involvement in human acute leukemia. Its Drosophila homolog trithorax (trx) is a member of the trx-Polycomb gene family, which maintains correct spatial expression of the Antennapedia and bithorax complexes during embryogenesis. The C-terminal SET domain of ALL-1 and TRITHORAX (TRX) is a 150-aa motif, highly conserved during evolution. We performed yeast two hybrid screening of Drosophila cDNA library and detected interaction between a TRX polypeptide spanning SET and the SNR1 protein. SNR1 is a product of snr1, which is classified as a trx group gene. We found parallel interaction in yeast between the SET domain of ALL-1 and the human homolog of SNR1, INI1 (hSNF5). These results were confirmed by in vitro binding studies and by demonstrating coimmunoprecipitation of the proteins from cultured cells and/or transgenic flies. Epitope-tagged SNR1 was detected at discrete sites on larval salivary gland polytene chromosomes, and these sites colocalized with around one-half of TRX binding sites. Because SNR1 and INI1 are constituents of the SWI/SNF complex, which acts to remodel chromatin and consequently to activate transcription, the interactions we observed suggest a mechanism by which the SWI/SNF complex is recruited to ALL-1/trx targets through physical interactions between the C-terminal domains of ALL-1 and TRX and INI1/SNR1.

ALL-1基因因参与人类急性白血病的发生而被发现。其果蝇（Drosophila）同源基因三胸蛋白（trithorax，trx）属于trx-Polycomb基因家族，该家族在胚胎发生过程中维持触角足复合体（Antennapedia complex）与双胸复合体（bithorax complex）的正确空间表达模式。ALL-1与TRITHORAX（TRX）的C端SET结构域（SET domain）是一段由150个氨基酸残基组成的基序，在进化过程中高度保守。我们对果蝇cDNA文库进行了酵母双杂交筛选，检测到一段包含SET结构域的TRX多肽与SNR1蛋白之间存在相互作用。SNR1是snr1基因的编码产物，该基因被归类为trx组基因。我们在酵母中还观测到，ALL-1的SET结构域与SNR1的人类同源蛋白INI1（hSNF5）之间存在类似的相互作用。上述结果经体外结合实验验证，并通过在培养细胞和/或转基因果蝇中免疫共沉淀检测到蛋白相互作用得到证实。带有表位标签的SNR1在幼虫唾液腺多线染色体的离散位点上被检测到，这些位点与约半数TRX结合位点共定位。由于SNR1与INI1均为SWI/SNF复合物（SWI/SNF complex）的组成成分，该复合物通过重塑染色质进而激活转录，我们观测到的相互作用提示了一种潜在机制：SWI/SNF复合物可通过ALL-1与TRX的C端结构域与INI1/SNR1之间的物理相互作用，被招募至ALL-1/trx的靶位点。