The proteins needed to localise key RNA molecules to the head end of a Drosophila egg (above) are moonlighting from their 'proper' job.

Almost 20 years ago, researchers discovered that messenger RNA (mRNA) from the bicoid gene accumulates at one end of the unfertilised Drosophila egg. The bicoid end always develops into the head. But how does bicoid mRNA come to be deposited so specifically at one end? Daniel St Johnston and Uwe Irion at the Wellcome Trust/Cancer Research UK Gurdon Institute in Cambridge have now found that the task is accomplished by a collection of proteins 'moonlighting' from their usual role in the cell.

The collection, known as ESCRT-II, was discovered recently as part of the cell's sorting and protein disposal machinery, which distributes proteins engulfed by the cell to different internal compartments for safe destruction.

Using a genetic screen to trawl for genes needed to localise bicoid mRNA, the Cambridge team found that the process depended on a gene coding for a protein found in the ESCRT-II complex. This protein sticks directly and specifically to the end of bicoid mRNA, which is then transported along the microtubule scaffold in the egg to one of the ends. The processes seem to be entirely distinct – evolution seems to have co-opted the ESCRT-II proteins for a new role in development.

Image credit: Daniel St Johnston

External links

• Irion U, St Johnston D. Bicoid RNA localization requires specific binding of an endosomal sorting complex. Nature 2007;445(7127):554–8.