Credit: G. S. WILKIE & I. DAVIS

The tops and bottoms of cells generally differ in shape and structure, as well as in their protein components. In embryos, such asymmetry is crucial in establishing the pattern of cells and tissues that will make up the adult organism. One way in which the asymmetric distribution of proteins is achieved is through their encoding messenger RNAs. Take, for example, the early fruitfly embryo, pictured here. Some mRNAs (red) form stripes along the top (apical part) of the monolayer of cells that constitutes this stage of development. Others are found at the bottom. Two groups, writing in Cell, now delve deeper into the asymmetric distribution of RNA in fruitflies.

Andrew J. Simmonds and colleagues (Cell 105, 197–207; 2001) show that the mRNA encoding a signalling protein, Wingless, is concentrated at the apical side of certain cells. Wingless is involved in many embryonic patterning processes, and, as Simmonds et al. show, if its mRNA is not distributed correctly the protein does not function properly. The authors also identify the 'address labels' in the mRNA that are crucial for its distribution.

Meanwhile, Gavin S. Wilkie and Ilan Davis (105, 209–219; 2001) have traced the journey of some apical mRNAs from where they are produced to their final destination. They find, first, that all mRNAs diffuse at random from their source in the nucleus. In other words, apical mRNAs do not necessarily leave the nucleus on the apical side.

Instead, once outside the nucleus, particles containing the apical mRNAs make their way rapidly towards the apical part of the cell. These particles are now transported specifically in the right direction — rather than, for example, diffusing randomly throughout the cell and becoming anchored only in the apical part. They are probably transported along microtubule-based tracks, with a motor protein known as dynein acting as the transport vehicle. It remains to be seen, however, how the address labels in the mRNAs connect with this transport machinery.