Structure and degradation mechanisms of 3′ to 5′ exoribonucleases

Exoribonucleases are enzymes that cleave RNA molecules by removing terminal nucleotides from the 3′ or 5′ end of the RNA molecules. They are key factors in RNA metabolism and have a relevant role in the processing and degradation of all types of RNAs. The 3′ to 5′ exoribonucleases are divided into families, according to their sequence and structural characteristics. The PDX family contains phosphate-dependent degradative enzymes, which can also perform the synthesis of RNA tails when phosphate is limiting. The RNB family contains hydrolytic enzymes with a similar domain organization. All proteins from this widespread family present the characteristic RNB domain responsible for the 3′ to 5′ exoribonuclease activity. In eukaryotes they can act alone or in a complex, the exosome, where they are the only active component. Finally, the DEDD family includes both RNA and DNA exonucleases and they present a similar mechanism of action. In this chapter, we will summarize the available information regarding the 3′ to 5′ exoribonucleases and discuss their importance for the RNA metabolism.