The recent discovery of microRNAs (miRNAs) has caused a great deal of interest, as they appear to be a large group of small RNAs (20–24 nucleotides in length), with an as yet unknown function. But Gideon Dreyfuss and colleagues might have taken the first step towards revealing the purpose of these enigmatic structures with their report in Genes & Development of a new class of ribonucleoprotein (RNP) complex that house several miRNAs.

The main components of the miRNP are Gemin3 and Gemin4 — components of the Survival Motor Neurons (SMN) complex — and the eukaryotic translation initiation factor eIF2C2. The eIF2C2–Gemin3–Gemin4 complex was isolated by immunoprecipitation studies designed to understand the physiological role of Gemin3, a DEAD-box putative RNA helicase. Further analysis of the complex revealed that an RNA pool of around 22 nucleotides could also be precipitated, and that the eIF2C2–Gemin3–Gemin4–miRNA complex sediments at around 15S.

After directionally cloning and sequencing this RNA pool, the authors identified nine miRNAs that had been previously discovered and 31 new miRNAs, which suggests that the number of miRNAs is much higher than previously thought. The sequences of the miRNAs show similarities to another type of small RNA — small temporal RNAs (stRNAs) — which suggests that the miRNAs are derived from larger precursors that have the capacity to form stem–loop structures, and that miRNAs are likely to regulate the expression of other RNAs.

The large number of miRNAs found within the complex suggests that miRNPs recognize a wide range of RNA targets, and identifying these targets will be crucial to understanding the miRNP function. But clues to the pathways and function of miRNAs can be also gained from knowing the functions of Gemin3, Gemin4 and eIF2C2.

The discovery of eIF2C2 — a member of the Argonaute protein family — in the miRNP confirms previous findings that linked these proteins with small RNA function. The presence of Gemin3 and Gemin4 is particularly intriguing as these are also found in the SMN complex, which is a key factor in the biogenesis and function of diverse RNPs. Interestingly, the binding of Gemin3 to the SMN protein (the component from which the SMN complex's name is derived) is impaired in SMN mutations that are found in patients with spinal muscular atrophy. So, further studies will be required to see what regulates the distribution of Gemin3 and Gemin4 between the SMN complex and miRNPs, and also whether there is a link between miRNPs and progression of this neurological disease.