Myosin-V regulates oskar mRNA localization in the Drosophila oocyte.

Intracellular mRNA localization is an effective mechanism for protein targeting leading to functional polarization of the cell. The mechanisms controlling mRNA localization and specifically how the actin and microtubule (MT) cytoskeletons cooperate in this process are not well understood. In Drosophila, Oskar protein accumulation at the posterior pole of the oocyte is required for embryonic development and is achieved by the transport of oskar mRNA and its exclusive translation at the posterior pole.

Drosophila PTB promotes formation of high-order RNP particles and represses oskar translation.

Local translation of asymmetrically enriched mRNAs is a powerful mechanism for functional polarization of the cell. In Drosophila, exclusive accumulation of Oskar protein at the posterior pole of the oocyte is essential for development of the future embryo. This is achieved by the formation of a dynamic oskar ribonucleoprotein (RNP) complex regulating the transport of oskar mRNA, its translational repression while unlocalized, and its translational activation upon arrival at the posterior pole.

A novel role for Gemin5 in mRNA translation.

In eukaryotic cells translation initiation occurs through two alternative mechanisms, a cap-dependent operating in the majority of mRNAs, and a 5'-end-independent driven by internal ribosome entry site (IRES) elements, specific for a subset of mRNAs. IRES elements recruit the translation machinery to an internal position in the mRNA through a mechanism involving the IRES structure and several trans-acting factors. Here, we identified Gemin5 protein bound to the foot-and-mouth disease virus (FMDV) and hepatitis C virus (HCV) IRES using two independent approaches, riboproteomic analysis and immunoprecipitation of photocrosslinked factors.

Relevance of RNA structure for the activity of picornavirus IRES elements.

The RNA of all members of the Picornaviridae family initiates translation internally, via an internal ribosome entry site (IRES) element present in their 5' untranslated region. IRES elements consist of cis-acting RNA structures that often operate in association with specific RNA-binding proteins to recruit the translational machinery. This specialized mechanism of translation initiation is shared with other viral RNAs, and represents an alternative to the general cap-dependent initiation mechanism.

Specific interference between two unrelated internal ribosome entry site elements impairs translation efficiency.

Internal ribosome entry site (IRES) elements allow simultaneous synthesis of multiple proteins in eukaryotic cells. Here, two unrelated IRESs that perform efficiently in bicistronic constructs, the picornavirus foot-and-mouth disease virus (FMDV) and the cellular immunoglobulin heavy chain binding protein (BiP) IRES, were used to generate a tricistronic vector. Functional analysis of the tricistronic RNA evidenced that the efficiency of protein synthesis under the control of BiP IRES was lower than that of the FMDV IRES, relative to the efficiency measured in bicistronic vectors.

Hrp48, a Drosophila hnRNPA/B homolog, binds and regulates translation of oskar mRNA.

Establishment of the Drosophila embryonic axes provides a striking example of RNA localization as an efficient mechanism for protein targeting within a cell. oskar mRNA encodes the posterior determinant and is essential for germline and abdominal development in the embryo. Tight restriction of Oskar activity to the posterior is achieved by mRNA localization-dependent translational control, whereby unlocalized mRNA is translationally repressed and repression is overcome upon mRNA localization.

IRES elements: features of the RNA structure contributing to their activity.

The activity of internal ribosome entry site (IRES) elements depends on their structural organization. We have addressed here the study of conserved structural motifs in the foot-and-mouth disease virus (FMDV) IRES as an example to understand the relationship between RNA structure and function. The features of the RNA structure known to be functionally relevant are discussed in regards to the capacity to modulate interaction of translation initiation factors with the FMDV IRES element.

IRES-driven translation is stimulated separately by the FMDV 3'-NCR and poly(A) sequences.

The 3' end region of foot-and-mouth disease virus (FMDV) consists of two distinct elements, a 90 nt untranslated region (3'-NCR) and a poly(A) tract. Removal of either the poly(A) tract or both the 3'-NCR and the poly(A) tract abrogated infectivity in susceptible cells in the context of a full-length cDNA clone. We have addressed the question of whether the impairment of RNA infectivity is related to defects at the translation level using a double approach.