Ribonuclease inhibitor
![Side view of porcine ribonuclease inhibitor;[1] ribbon is colored from blue (N-terminus) to red (C-terminus).](/uploads/202502/07/2bnh_sideview5046.png)
Ribonuclease inhibitor (RI) is a large (~450 residues, ~49 kDa), acidic (pI ~4.7), leucine-rich repeat protein that forms extremely tight complexes with certain ribonucleases. It is a major cellular protein, comprising ~0.1% of all cellular protein by weight, and appears to play an important role in regulating the lifetime of RNA.