Building specificity with nonspecific RNA-binding proteins

Specificity is key to biological regulation. Two families of RNA binding proteins, heterogeneous nuclear ribonucleoproteins and serine-arginine–rich proteins, were initially thought to have redundant or nonspecific biochemical functions. Recently, members of these families have been found as components of distinct regulatory complexes with highly specific and essential roles in mRNA metabolism. Here we discuss the basis for their functional specificity and the mechanisms of action of some of their characteristic protein domains.