Nuclear transit of the RNA-binding protein She2 is required for translational control of localized ASH1 mRNA.

Cytoplasmic localization and localized translation of messenger RNAs contribute to asymmetrical protein distribution. Recognition of localized mRNAs by RNA-binding proteins can occur in the cytoplasm or, alternatively, co- or post-transcriptionally in the nucleus. In budding yeast, mRNAs destined for localization are bound by the She2 protein before their nuclear export. Here, we show that a specific transcript, known as ASH1 mRNA, and She2 localize specifically to the nucleolus when their nuclear export is blocked. Nucleolar She2 localization is enhanced in a She2 mutant that cannot bind to RNA. A fusion protein of the amino terminus of She3 and She2 (She3N-She2) fails to enter the nucleus, but does not impair ASH1 mRNA localization. Instead, these cells fail to distribute Ash1 protein asymmetrically, which is caused by a defective translational control of ASH1 mRNA. Our results indicate that the nucleolar transit of RNA-binding proteins such as She2 is necessary for the correct assembly of translationally silenced localizing messenger ribonucleoproteins.

A putative function of the nucleolus in the assembly or maturation of specialized messenger ribonucleoprotein complexes.

The nucleolus is a nuclear subcompartment with a well-defined function in ribosomal RNA transcription and assembly of ribosomal subunits. However, the nucleolus is multifunctional and involved in processes as diverse as regulation of mitosis, cell proliferation and viral infection. There is increasing evidence that, in addition to ribosomes, several different ribonucleoprotein (RNP) particles assemble or mature in the nucleolus, including the signal recognition particle (SRP) or viral RNPs. Here we discuss recent findings from mammalian and yeast cells that specific localized mRNAs or proteins that associate with localized mRNAs can accumulate in the nucleolus under particular conditions. Experimental evidence derived from studies on the nucleolar accumulation of yeast ASH1 mRNA and of cytoplasmic retention of its binding protein She2 are integrated into a hypothetical model that suggests a novel role of the nucleolus in the assembly of specific mRNPs.

Monomeric myosin V uses two binding regions for the assembly of stable translocation complexes.

Myosin-motors are conserved from yeast to human and transport a great variety of cargoes. Most plus-end directed myosins, which constitute the vast majority of all myosin motors, form stable dimers and interact constitutively with their cargo complexes. To date, little is known about regulatory mechanisms for cargo-complex assembly. In this study, we show that the type V myosin Myo4p binds to its cargo via two distinct binding regions, the C-terminal tail and a coiled-coil domain-containing fragment. Furthermore, we find that Myo4p is strictly monomeric at physiologic concentrations. Because type V myosins are thought to require dimerization for processive movement, a mechanism must be in place to ensure that oligomeric Myo4p is incorporated into cargo-translocation complexes. Indeed, we find that artificial dimerization of the Myo4p C-terminal tail promotes stabilization of myosin-cargo complexes, suggesting that full-length Myo4p dimerizes in the cocomplex as well. We also combined the Myo4p C-terminal tail with the coiled-coil region, lever arm, and motor domain from a different myosin to form constitutively dimeric motor proteins. This heterologous motor successfully translocates its cargo in vivo, suggesting that wild-type Myo4p may also function as a dimer during cargo-complex transport.