RNA/ssDNA-binding proteins comprise an emerging class of multifunctional proteins with an anticipated role in coupling transcription with RNA processing. We focused here on the highly related transcription factors of the TET sub-class: TLS/FUS, EWS and in particular the least studied member TAF15. An extensive array of immunoprecipitation studies on differentially extracted HeLa nuclei revealed the specific association of TAF15 with the spliceosomal U1 snRNP complex, as deduced by the co-precipitating U1 snRNA, U1-70K and Sm proteins. Additionally, application of anti-U1 RNP autoantibodies identified TAF15 in the immunoprecipitates. Minor fractions of nuclear TAF15 and U1 snRNP were involved in this association. Pull-down assays using recombinant TAF15 and U1 snRNP-specific proteins (U1-70K, U1A and U1C) provided in vitro evidence for a direct protein-protein interaction between TAF15 and U1C, which required the N-terminal domain of TAF15. The ability of TAF15 to directly contact RNA, most likely RNA pol II transcripts, was supported by in vivo UV cross-linking studies in the presence of α-amanitin. By all findings, the existence of a functionally discrete subset of U1 snRNP in association with TAF15 was suggested and provided further support for the involvement of U1 snRNP components in early steps of coordinated gene expression.
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