The composition of Staufen-containing RNA granules from human cells indicates their role in the regulated transport and translation of messenger RNAs

Abstract

hStaufen is the human homolog of dmStaufen, a double-stranded (ds)RNA-binding protein involved in early development of the fly. hStaufen-containing complexes were purified by affinity chromatography from human cells transfected with a TAP-tagged hStaufen gene. These complexes showed a size >10 MDa. Untagged complexes with similar size were identified from differentiated human neuroblasts. The identity of proteins present in purified hStaufen complexes was determined by mass spectrometry and the presence of these proteins and other functionally related ones was verified by western blot. Ribosomes and proteins involved in the control of protein synthesis (PABP1 and FMRP) were present in purified hStaufen complexes, as well as elements of the cytoskeleton (tubulins, tau, actin and internexin), cytoskeleton control proteins (IQGAP1, cdc42 and rac1) and motor proteins (dynein, kinesin and myosin). In addition, proteins normally found in the nucleus, like nucleolin and RNA helicase A, were also found associated with cytosolic hStaufen complexes. The co-localization of these components with hStaufen granules in the dendrites of differentiated neuroblasts, determined by confocal immunofluorescence, validated their association in living cells. These results support the notion that the hStaufen-containing granules are structures essential in the localization and regulated translation of human mRNAs in vivo.

Staufen1 senses overall transcript secondary structure to regulate translation

“…RHA is a positive regulator of translation on mRNAs containing 5′-UTR secondary structures 43 and is known to copurify with Stau1 (ref. 44 and E.P.R., unpublished data). Another abundant translational-regulatory protein that binds ribosomes and cross-links throughout CDS regions is the fragile X protein, FMRP 45 .…”

The RNA binding and transport proteins staufen and fragile X mental retardation protein are expressed by rat primary afferent neurons and localize to peripheral and central axons

“…Two of the best characterized RNA binding and transport proteins are stau and FMRP, which coexist in RNA/protein granules (Ohashi et al, 2002;Kanai et al, 2004;Villace et al, 2004). Stau is involved in RNA transport to synaptic sites in neurons (Kiebler et al, 1999;Tang et al, 2001) and appears to be involved in both learning and memory (Dubnau et al, 2003) and spine morphogenesis (Goetze et al, 2006).…”

“…Taken together, these results indicate that DRG and TG neurons express stau and FMRP, proteins thought to be involved in RNA transport and local regulation of translation. These proteins are found in RNA granules that localize to distal sites in CNS neurons (Ohashi et al, 2002;Kanai et al, 2004;Villace et al, 2004), suggesting that they may serve a similar function in axons of the PNS. …”

A dual role for IQGAP1 in regulating exocytosis

“…In mammals, the Sec61␤ subunit that associates with IQGAP1 and the exocyst binds non-translating ribosomes (Levy et al, 2001). By contrast, IQGAP1 and CDC42, but not other effectors such as WASP, were purified as bindingpartners for the double-stranded RNA-binding polarity protein Staufen as part of RNA granules involved in the in vivo localization and translation of human mRNAs (Villace et al, 2004). Future investigation should reveal whether IQGAP1 tethers ribosomes to mRNAs, serving as a scaffolding positional marker.…”