A Human Sequence Homologue of Staufen Is an RNA-Binding Protein That Is Associated with Polysomes and Localizes to the Rough Endoplasmic Reticulum

Abstract

ABSTRACT In the course of a two-hybrid screen with the NS1 protein of influenza virus, a human clone capable of coding for a protein with high homology to the Staufen protein from Drosophila melanogaster (dmStaufen) was identified. With these sequences used as a probe, cDNAs were isolated from a λ 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 coli as a His-tagged polypeptide. It showed dsRNA binding activity 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 an apparent molecular mass of 60 to 65 kDa. The intracellular localization of hStaufen-like protein was investigated by immunofluorescence using a series of markers for the cell compartments. Colocalization was observed with the rough endoplasmic reticulum but not with endosomes, cytoskeleton, or Golgi apparatus. Furthermore, sedimentation analyses indicated that hStaufen-like protein associates with polysomes. These results are discussed in relation to the possible functions of the protein.