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

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 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.The establishment and maintenance of asymmetries in certain cells implies the localized expression of many proteins, a property often enhanced by the localization of the corresponding mRNAs (for reviews, see references 2 and 47). The relevance of mRNA localization at precise sites of the cell in the definition of polarity of developing embryos has been documented in both Drosophila melanogaster and Xenopus laevis. Thus, the positions of gurken and oskar mRNAs in the fly oocyte define its dorsoventral axis (36) and the location of the pole plasm at the posterior pole (15), respectively. Likewise, the localization of the bicoid and nanos mRNAs at the anterior and posterior poles of the embryo, respectively, leads to the generation of two opposing gradients of their protein products and ultimately to the definition of the head, thorax, and abdomen of the embryo (50). In the case of X. laevis, several mRNAs, such as Xcat2, Xcat3, and Xlsirt, are directed to the germ plasm (34), while others, such as Vg1 and Xwnt11, accumulate at the vegetal cortex (30). By analogy to the D. melanogaster genes, these mRNAs are thought to play a role in defining patterns in the X. laevis embryo. In fact, a region of the Xcat2 protein shows sequence homology to the zinc finger domain of nanos protein (34).The specific localization of certain mRNAs to precise sites within the cell is not an exclusive property of germ cells or developing embryos. A number of observations indicate that some of the mRNAs of somatic cells are also localized at different sites within the cell. Thus, myelin-binding protein is translated in oligodendrocytes from free ribosomes on localized mRNA (51), and the microtubule-associated protein MAP2 is translated preferentially i...

Staufen1 senses overall transcript secondary structure to regulate translation

“…Bioinformatics analysis of the footprints, however, failed to identify any particular enriched motif (A.K. and E.P.R., unpublished data), results consistent with the idea that Staufen recognizes dsRNA in a sequence-independent manner [39][40][41] .…”

Insights into mRNA transport in neurons

“…Finally, the isolation of Staufencontaining particles by Mallardo et al (19) has confirmed the previous suggestion that there are two pools of both Staufen 1 and 2 in mammalian neurons (24,25). One pool fractionates with endoplasmic reticulum (ER) in their biochemical experiments and may represent Staufen observed in large granules that colocalize with the rough ER within the soma or at synaptic terminals (22,26,28,32,33 …”

“…Recently, two homologues of Staufen have been identified in mammals: Staufen 1 (22)(23)(24)(25)(26) and Staufen 2 (27)(28)(29). Both Staufen 1 and 2 are expressed in the nervous system; both proteins contain several dsRBDs and have a tubulin-binding domain (22)(23)(24)(25)(26)(27)(28)(29). What role could Staufen be playing in the mammalian nervous system?…”

Isolation and characterization of Staufen-containing ribonucleoprotein particles from rat brain

“…To study these RNA-containing particles in detail, we set out to biochemically isolate these complexes on the basis of two criteria. First, we chose Stau1 (13,23,24) and Stau2 (16,17) proteins, as biochemical markers to follow their isolation. Second, we assumed based on an attractive model (10) that the observed Staufen-containing moving particles represent soluble RNPs not associated with membranes.…”