Targeting messenger RNA (mRNA) transcripts to the appropriate subcellular location provides a mechanism for optimizing production of a particular protein at the right place and the right time. Zipcode binding protein 1 (ZBP1) binds to the "zipcode" sequence in the β-actin mRNA transcript, promoting its translocation to sites of actin polymerization, such as growth cones or the leading edge of migrating fibroblasts. Hüttelmaier et al. found that, in differentiated NG108-15 neuroblastoma cells, ZBP1 labeled with green fluorescent protein colocalized with β-actin mRNA in the nucleus as well as at the leading edge, along neurites, and in filopodia. ZBP1 inhibited translation of β-actin in rabbit reticulocyte lysates and, in cells, ZBP1 knockdown with siRNA enhanced translation of a reporter containing the β-actin 3′ region in a manner that depended on the presence of the zipcode sequence. Sequence analysis indicated that ZBP1 contained a possible SH3-binding motif that might function as a Src tyrosine kinase docking site. Indeed, Src phosphorylated ZBP1 on Tyr 396 both in vitro and in cells. Increased Src activity promoted translation of a zipcode-containing reporter, and analysis of nonphosphorylatable or phosphorylation-mimicking mutants indicated that ZBP1 phosphorylation decreased its association with β-actin mRNA. FRET analysis indicated that Src associated with ZBP1 in growth cones and near filopodia. ZBP1 knockdown interfered with neurite outgrowth; this could be rescued by phosphorylatable ZBP1 but not by the nonphosphorylatable mutant. Thus, the authors conclude that ZBP1 not only regulates β-actin localization but also prevents its premature translation until reaching an appropriate locale and undergoing phosphorylation by Src. Dahm and Kiebler provide context in a News and Views article.
S. Hüttelmaier, D. Zenklusen, M. Lederer, J. Dictenberg, M. Lorenz, X. Meng, G. J. Bassell, J. Condeelis, R. H. Singer, Spatial regulation of β-actin translation by Src-dependent phosphorylation by ZBP1. Nature 438, 512-515 (2005). [PubMed]
R. Dahm, M. Kiebler, Silenced mRNA on the move. Nature 438, 432-435 (2005). [PubMed]