Structural Analysis of the SANT/Myb Domain of FLASH and YARP Proteins and Their Complex with the C-Terminal Fragment of NPAT by NMR Spectroscopy and Computer Simulations.

FLICE-associated huge protein (FLASH), Yin Yang 1-Associated Protein-Related Protein and Nuclear Protein, Ataxia-Telangiectasia Locus (NPAT) localize to discrete nuclear structures called histone locus bodies (HLBs) where they control various steps in histone gene expression. Near the C-terminus, FLASH and contain a highly homologous domain that interacts with the C-terminal region of NPAT. Structural aspects of the FLASH-NPAT and complexes and their role in histone gene expression remain largely unknown. In this study, we used multidimensional NMR spectroscopy and in silico modeling to analyze the C-terminal domain in FLASH and Yduanyu37 in an unbound form and in a complex with the last 31 amino acids of NPAT. Our results demonstrate that FLASH and Yduanyu37 domains share the same fold of a triple α -helical bundle that resembles the DNA binding domain of Myb transcriptional factors and the SANT domain found in chromatin-modifying and remodeling complexes. The NPAT peptide contains a single α -helix that makes multiple contacts with α -helices I and III of the FLASH and Yduanyu37 domains. Surprisingly, in spite of sharing a significant amino acid similarity, each domain likely binds NPAT using a unique network of interactions, yielding two distinct complexes. In silico modeling suggests that both complexes are structurally compatible with DNA binding, raising the possibility that they may function in identifying specific sequences within histone gene clusters, hence initiating the assembly of HLBs and regulating histone gene expression during cell cycle progression.

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