TLS (FUS) BINDS RNA IN-VIVO AND ENGAGES IN NUCLEOCYTOPLASMIC SHUTTLING

TLS, the product of a gene commonly translocated in liposarcomas (TLS) , is prototypical of a newly identified class of nuclear proteins that contain a C-terminal domain with a distinct RNA recognition motif (RR M) surrounded by Arg-Gly-Gly (RGG) repeats. Its unique N terminus serv es as an essential transforming domain for a number of fusion oncoprot eins in human sarcomas and leukemias. In this study we use an in vivo UV crosslinking procedure to probe the interactions of TLS with RNA, T LS is found to bind RNA in vivo and the association of TLS with RNA is rapidly diminished by treating cells with transcriptional inhibitors, This suggests that the species bound by TLS turns over rapidly, Surpr isingly, the RRIM was found to be dispensable for RNA binding by TLS i n vivo, suggesting that at any one time most of the interactions betwe en TLS and RNA in the cell are not sequence specific, Analysis of inte r specific heterokaryons formed between human and mouse or Xenopus cel ls revealed that TLS engages in rapid nucleocytoplasmic shuttling, a f inding confirmed by the ability of anti-TLS antibodies to trap TLS whe n injected into the cytoplasm of HeLa cells, Cellular fractionation ex periments suggest that TLS binds to RNA in both the nucleus and cytopl asm and support the hypothesis that TLS functions as a heterogeneous r ibonuclear protein (hnRNP)-like chaperone of RNA. These findings are d iscussed in the context of the role altered forms of TLS play in cellu lar transformation.