Translocated in liposarcoma (TLS) is an importantprotein component of the heterogeneous nuclear ribonucleoprotein complex involved in the splicing of pre-mRNA and the export of fully processed mRNA to the cytoplasm. We examined the domain organization of human TLS by a combined approach using limited proteolysis, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, circular dichroism, inductively coupled plasma atomic emission spectroscopy, and NMR spectroscopy. We found that the RNA recognition motif (RRM) and zinc finger-like domains exclusively form protease-resistant core structures within the isolated TLS protein fragments, while the remaining regions, including the Arg-Gly-Gly repeats, appear to be completely unstructured. Thus, TLS contains the unstructured N-terminal half followed by the RRM and zinc finger-like domains, which are connected to each other by a flexible linker. We also carried out NMR analyses to obtain more detailed insights into the individual RRM and zinc finger-like domains. The 113 Cd NMR analysis of the zinc finger-like domain verified that zinc is coordinated with four cysteines in the C4 type scheme. We also investigated the interaction of each domain with an oligo-RNA containing the GGUG sequence, which appears to be critical for the TLS function in splicing. The backbone amide NMR chemical shift perturbation analyses indicated that the zinc finger domain binds GGUG-containing RNA with a dissociation constant of about 1.0 ؋ 10 ؊5 M, whereas the RRM domain showed no observable interaction with this RNA. This surprising result implies that the zinc finger domain plays a more predominant role in RNA recognition than the RRM domain.The translocated in liposarcoma (TLS) 1 protein, also termed FUS, was first identified in human myxoid and round cell liposarcomas as an oncogenic fusion protein with a stressinduced DNA-binding transcription factor, CCAAT enhancerbinding homologous protein (CHOP, also known as GADD153 or DDIT3) (1, 2). The resultant fusion protein (TLS-CHOP), consisting of the N-terminal half of TLS and the full-length CHOP, appears to act as a potent transcription factor possibly by combining the TLS transactivation activity and the CHOP DNA binding activity. A different type of fusion protein, TLS-ERG (a member of the erythroblast transformation-specific (ETS) family of transcription factors), was subsequently detected in human acute myeloid leukemia (3).The normal TLS, consisting of 526 amino acids with a calculated molecular mass of 53 kDa, belongs to a family including the closely related proteins Ewing's sarcoma (EWS) (4) and TAF II 68 (TATA-binding protein-associated factor) (5). Thus, they are collectively called the TET (TLS, EWS, TAF II 68) family. EWS and TAF II 68 interact with components of the RNA polymerase II complex (5, 6). Moreover sarcoma-associated RNA-binding fly homologue (SARFH), a Drosophila homologue of TLS, is colocalized with the polymerase on active chromatin (7). The N-terminal domain of TLS is involved in transcription...