Association of human DEAD box protein DDX1 with a cleavage stimulation factor involved in 3 '-end processing of pre-mRNA

DEAD box proteins are putative RNA helicases that function in all aspects o f RNA metabolism, including translation, ribosome biogenesis, and pre-mRNA splicing. Because many processes involving RNA metabolism are spatially org anized within the cell, we examined the subcellular distribution of a human DEAD box protein, DDX1, to identify possible biological functions. Immunof luorescence labeling of DDX1 demonstrated that in addition to widespread pu nctate nucleoplasmic labeling, DDX1 is found in discrete nuclear foci simil ar to0.5 mum in diameter. Costaining with anti-Sm and anti-promyelocytic le ukemia (PML) antibodies indicates that DDX1 foci are frequently located nex t to Cajal (coiled) bodies and less frequently, to PML bodies. Most importa ntly, costaining with anti-CstF-64 antibody indicates that DDX1 foci coloca lize with cleavage bodies. By microscopic fluorescence resonance energy tra nsfer, we show that labeled DDX1 resides within a Forster distance of 10 nm of labeled CstF-64 protein in both the nucleoplasm and within cleavage bod ies. Coimmunoprecipitation analysis indicates that a proportion of CstF-64 protein resides in the same complex as DDX1. These studies are the first to identify a DEAD box protein associating with factors involved in T-end cle avage and polyadenylation of pre-mRNAs.