Ribonuclease (commonly abbreviated RNase) is a type of nuclease that catalyzes the degradation of RNA into smaller components. Ribonucleases can be divided into endoribonucleases and exoribonucleases, and comprise several sub-classes within the EC 2.7 (for the phosphorolytic enzymes) and 3.1 (for the hydrolytic enzymes) classes of enzymes.
Function
All organisms studied contain many RNases of two different classes, showing that RNA degradation is a very ancient and important process. As well as clearing of cellular RNA that is no longer required, RNases play key roles in the maturation of all RNA molecules, both messenger RNAs that carry genetic material for making proteins and non-coding RNAs that function in varied cellular processes. In addition, active RNA degradation systems are the first defense against RNA viruses and provide the underlying machinery for more advanced cellular immune strategies such as RNAi.
Some cells also secrete copious quantities of non-specific RNases such as A and T1. RNases are, therefore, extremely common, resulting in very short lifespans for any RNA that is not in a protected environment. It is worth noting that all intracellular RNAs are protected from RNase activity by a number of strategies including 5' end capping, 3' end polyadenylation, formation of an RNA·RNA duplex, and folding within an RNA protein complex (ribonucleoprotein particle or RNP).
Another mechanism of protection is ribonuclease inhibitor (RI), which comprises a relatively large fraction of cellular protein (~0.1%) in some cell types, and which binds to certain ribonucleases with the highest affinity of any protein-protein interaction; the dissociation constant for the RI-RNase A complex is ~20 fM under physiological conditions. RI is used in most laboratories that study RNA to protect their samples against degradation from environmental RNases.
Similar to restriction enzymes, which cleave highly specific sequences of double-stranded DNA, a variety of endoribonucleases that recognize and cleave specific sequences of single-stranded RNA have been recently classified.
RNases play a critical role in many biological processes, including angiogenesis and self-incompatibility in flowering plants (angiosperms). Many stress-response toxins of prokaryotic toxin-antitoxin systems have been shown to have RNase activity and homology.