A human sequence homologue of Staufen is an RNA-binding protein that is associated with polysomes and localizes to the rough endoplasmic reticulum

In the course of a two-hybrid screen with the NS1 protein of influenza viru s, a human clone capable of coding for a protein with high homology to the Staufen protein from Drosophila melanogaster (dmStaufen) was identified. Wi th these sequences used as a probe, cDNAs were isolated from a lambda cDNA library. The encoded protein (hStaufen-like) contained four double-stranded RNA (dsRNA)-binding domains with 55% similarity and 38% identity to those of dmStaufen, including identity at all residues involved in RNA binding. A recombinant protein containing all dsRNA-binding domains was expressed in Escherichia call as a His-tagged polypeptide. It showed dsRNA binding activ ity in vitro, with an apparent K-d of 10(-9) M. Using a specific antibody, we detected in human cells a major form of the hStaufen-like protein with a n apparent molecular mass of 60 to 65 kDa. The intracellular localization o f hStaufen-like protein was investigated by immunofluorescence using a seri es of markers for the cell compartments. Colocalization was observed with t he rough endoplasmic reticulum but not with endosomes, cytoskeleton, or Gol gi apparatus. Furthermore, sedimentation analyses indicated that hStaufen-l ike protein associates with polysomes. These results are discussed in relat ion to the possible functions of the protein.