Some might recall playing with Mr. Potato Head as a child (or maybe earlier this morning). By sticking different noses, mouths, eyes, shoes, or other accessories on the basic potato head module, you could generate a variety of silly-looking spuds. Similarly, the interaction of different accessory proteins with the miRNA-argonaute complex might determine the fate of the targeted mRNA, as suggested recently in a Point of View by Christopher Hammell of the University of Massachusetts Medical School.
Argonaute (Ago) proteins associate directly with miRNA and are necessary for miRNA function. By binding to the m7G cap structure of mRNA targets, Agos can repress translation. However, miRNA−Ago complexes also appear to inhibit the translation of mRNA targets by other mechanisms, as well as influence mRNA localization and stability. In addition, recent reports indicate that under certain environmental conditions (i.e., serum starvation), miRNA-Ago can actually enhance mRNA translation. The mechanisms by which this complex mediates such diverse processes are unknown.
Hammell hypothesizes that each miRNA-Ago complex serves as a scaffold that can accomplish minimal mRNA repression, and various accessory factors modulate this activity. For example, the fragile-X-mental-retardation-related protein 1 (FXR1) has been shown to associate with miRNA-Ago complexes that enhance the translation of target mRNAs. In this way, miRNA activities can be fine-tuned or changed completely depending on cell type, developmental stage, or physiological conditions. Check it out at RNA Biololgy, September 2008