DEAD box proteins are putative RNA helicases that function in all aspects of RNA 
metabolism, including translation, ribosome biogenesis, and pre-mRNA splicing. 
Because many processes involving RNA metabolism are spatially organized within 
the cell, we examined the subcellular distribution of a human DEAD box protein, 
DDX1, to identify possible biological functions. Immunofluorescence labeling of 
DDX1 demonstrated that in addition to widespread punctate nucleoplasmic 
labeling, DDX1 is found in discrete nuclear foci approximately 0.5 microm in 
diameter. Costaining with anti-Sm and anti-promyelocytic leukemia (PML) 
antibodies indicates that DDX1 foci are frequently located next to Cajal 
(coiled) bodies and less frequently, to PML bodies. Most importantly, costaining 
with anti-CstF-64 antibody indicates that DDX1 foci colocalize with cleavage 
bodies. By microscopic fluorescence resonance energy transfer, we show that 
labeled DDX1 resides within a Förster distance of 10 nm of labeled CstF-64 
protein in both the nucleoplasm and within cleavage bodies. 
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 3'-end cleavage and 
polyadenylation of pre-mRNAs.