Spatial regulation of β-actin translation by Src-dependent phosphorylation of ZBP1

Abstract

Localization of beta-actin messenger RNA to sites of active actin polymerization modulates cell migration during embryogenesis, differentiation and possibly carcinogenesis. This localization requires the oncofetal protein ZBP1 (Zipcode binding protein 1), which binds to a conserved 54-nucleotide element in the 3'-untranslated region of the beta-actin mRNA known as the 'zipcode'. ZBP1 promotes translocation of the beta-actin transcript to actin-rich protrusions in primary fibroblasts and neurons. It is not known how the ZBP1-RNA complex achieves asymmetric protein sorting by localizing beta-actin mRNA. Here we show that chicken ZBP1 modulates the translation of beta-actin mRNA. ZBP1 associates with the beta-actin transcript in the nucleus and prevents premature translation in the cytoplasm by blocking translation initiation. Translation only occurs when the ZBP1-RNA complex reaches its destination at the periphery of the cell. At the endpoint of mRNA transport, the protein kinase Src promotes translation by phosphorylating a key tyrosine residue in ZBP1 that is required for binding to RNA. These sequential events provide both temporal and spatial control over beta-actin mRNA translation, which is important for cell migration and neurite outgrowth.

Expression of the RNA-binding protein IMP1 correlates with poor prognosis in ovarian carcinoma

“…In Xenopus, the ortholog Vg1-RBP is essential for the control of neuronal crest cell migration (Yaniv et al, 2003). In primary mammalian neurons as well as neuroblastoma cells, IMP1, also termed ZBP1 (zipcode-binding protein 1) regulates localized translation of the b-actin mRNA in growth cones and neurites supporting an important role of IMP proteins for guiding normal development and directing cell migration (Huttelmaier et al, 2005).…”

“…Although additional target mRNAs of IMP1 are expected, solid evidence indicates that IMP1 is essential for the regulation of c-myc, bTrCP1 and CD44 mRNA turnover, but also facilitates translational control of the IGF-II and b-actin mRNAs (Nielsen et al, 1999;Lemm and Ross, 2002;Huttelmaier et al, 2005;Noubissi et al, 2006;Vikesaa et al, 2006). While by the regulation of IGF-II and c-myc expression IMP1 is expected to affect cell proliferation, growth and survival, the regulation of CD44 or b-actin expression via IMP1 is suggested to modulate cell adhesion, invadopodia formation and actin dynamics.…”

“…To evaluate the role of IMP1 in ovarian carcinomas, phenotypic effects of aberrant IMP1 expression were analysed by small interfering RNA (siRNA)-mediated Immunostaining was performed on paraffin-embedded tissue sections, as described previously (Kobel et al, 2006), using a IMP1 (ZBP1)-directed antibody (Farina et al, 2003;Huttelmaier et al, 2005). Two slides per sample were immunostained together in two runs before the semiquantitative evaluation of the immunoreactivity by a pathologist (MK) blinded to the clinical data.…”

“…(a) Cell viability was determined in ES-2 or OVCAR-3 cells at indicated time points after IMP1-directed siRNA transfection and normalization to cell viability in a population transfected with control siRNAs. The siRNAs used (one siRNA directed against IMP1 and one control siRNA) were as described previously (Huttelmaier et al, 2005). (b) ES-2 cell numbers were determined at indicated time points after siRNA transfection and normalization to cell numbers at 24 h post-transfection (100%).…”

“…In (b), ES-2 cell numbers were determined post-transfection of indicated siRNAs and normalization to cell numbers 24 h post-transfection, set to 100%. The specific knockdown of IMP1 was determined by western blotting of OVCAR-3 and ES-2 cell lysates 72 h post-transfection with anti-vinculin (Sigma, Mu¨nchen, Germany), anti-tubulin (Sigma) and anti-IMP1 (ZBP1) (Farina et al, 2003), as described previously (Huttelmaier et al, 2005). Relative expression levels of indicated genes were determined by quantitative RT-PCR (d).…”