Ribonuclease inhibitor explained

Symbol:	LRR_1
Leucine Rich Repeat
Pfam:	PF00560
Pfam Clan:	CL0022
Interpro:	IPR003590
Smart:	SM00368
Scop:	1bnh
Pdb:	,,,,,

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.^[1]

RI has a surprisingly high cysteine content (~6.5%, cf. 1.7% in typical proteins) and is sensitive to oxidation. RI is also rich in leucine (21.5%, compared to 9% in typical proteins) and commensurately lower in other hydrophobic residues, esp. valine, isoleucine, methionine, tyrosine, and phenylalanine.

Structure

RI is the classic leucine-rich repeat protein, consisting of alternating α-helices and β-strands along its backbone. These secondary structure elements wrap around in a curved, right-handed solenoid that resembles a horseshoe. The parallel β-strands and α-helices form the inner and outer wall of the horseshoe, respectively. The structure appears to be stabilized by buried asparagines at the base of each turn, as it passes from α-helix to β-strand. The αβ repeats alternate between 28 and 29 residues in length, effectively forming a 57-residue unit that corresponds to its genetic structure (each exon codes for a 57-residue unit).

Binding to ribonucleases

The affinity of RI for ribonucleases is among the highest for any protein-protein interaction; the dissociation constant of the RI-RNase A complex is in the femtomolar (fM) range under physiological conditions. Despite this high affinity, RI is able to bind a wide variety of RNases A despite their relatively low sequence identity. Both biochemical studies and crystallographic structures of RI-RNase A complexes suggest that the interaction is governed largely by electrostatic interactions, but also involves substantial buried surface area.^{[2] [3]} RI's affinity for ribonucleases is important, since many ribonucleases have cytotoxic and cytostatic effects that correlate well with ability to bind RI.^[4]

Mammalian RIs are unable to bind certain pancreatic ribonuclease family members from other species. In particular, amphibian RNases, such ranpirnase and amphinase from the Northern leopard frog, escape mammalian RI and have been noted to have differential cytotoxicity against cancer cells.^[5]

See also

• Guanidinium thiocyanate - a chemical RNase inhibitor.

Further reading

• Kobe B, Deisenhofer J . A structural basis of the interactions between leucine-rich repeats and protein ligands . Nature . 374 . 6518 . 183–6 . Mar 1995 . 7877692 . 10.1038/374183a0 . 1995Natur.374..183K . 4364436 .
• Kobe B, Deisenhofer J . Mechanism of ribonuclease inhibition by ribonuclease inhibitor protein based on the crystal structure of its complex with ribonuclease A . Journal of Molecular Biology . 264 . 5 . 1028–43 . Dec 1996 . 9000628 . 10.1006/jmbi.1996.0694 .
• Papageorgiou AC, Shapiro R, Acharya KR . Molecular recognition of human angiogenin by placental ribonuclease inhibitor--an X-ray crystallographic study at 2.0 A resolution . The EMBO Journal . 16 . 17 . 5162–77 . Sep 1997 . 9311977 . 1170149 . 10.1093/emboj/16.17.5162 .
• Suzuki M, Saxena SK, Boix E, Prill RJ, Vasandani VM, Ladner JE, Sung C, Youle RJ . Engineering receptor-mediated cytotoxicity into human ribonucleases by steric blockade of inhibitor interaction . Nature Biotechnology . 17 . 3 . 265–70 . Mar 1999 . 10096294 . 10.1038/7010 . 23140257 .
• Shapiro R, Ruiz-Gutierrez M, Chen CZ . Analysis of the interactions of human ribonuclease inhibitor with angiogenin and ribonuclease A by mutagenesis: importance of inhibitor residues inside versus outside the C-terminal "hot spot" . Journal of Molecular Biology . 302 . 2 . 497–519 . Sep 2000 . 10970748 . 10.1006/jmbi.2000.4075 .
• Bretscher LE, Abel RL, Raines RT . A ribonuclease A variant with low catalytic activity but high cytotoxicity . The Journal of Biological Chemistry . 275 . 14 . 9893–6 . Apr 2000 . 10744660 . 10.1074/jbc.275.14.9893 . free .
• Yakovlev GI, Mitkevich VA, Makarov AA . Ribonuclease inhibitors . Molecular Biology . 40 . 6 . 867–874 . 2006 . 10.1134/S0026893306060045 . 31887913 .

Notes and References

1. Book: Shapiro R . Ribonucleases - Part A . Cytoplasmic ribonuclease inhibitor . Methods in Enzymology . 341 . 611–28 . 2001 . 11582809 . 10.1016/S0076-6879(01)41180-3 . 9780121822422 .
2. Lee FS, Shapiro R, Vallee BL . Tight-binding inhibition of angiogenin and ribonuclease A by placental ribonuclease inhibitor . Biochemistry . 28 . 1 . 225–30 . Jan 1989 . 2706246 . 10.1021/bi00427a031.
3. Papageorgiou AC, Shapiro R, Acharya KR . Molecular recognition of human angiogenin by placental ribonuclease inhibitor--an X-ray crystallographic study at 2.0 A resolution . The EMBO Journal . 16 . 17 . 5162–77 . Sep 1997 . 9311977 . 10.1093/emboj/16.17.5162 . 1170149.
4. Makarov AA, Ilinskaya ON . Cytotoxic ribonucleases: molecular weapons and their targets . FEBS Letters . 540 . 1–3 . 15–20 . April 2003 . 12681476 . 10.1016/s0014-5793(03)00225-4. 30324366 . free . 2003FEBSL.540...15M .
5. Ardelt W, Shogen K, Darzynkiewicz Z . Onconase and amphinase, the antitumor ribonucleases from Rana pipiens oocytes . Current Pharmaceutical Biotechnology . 9 . 3 . 215–25 . Jun 2008 . 18673287 . 10.2174/138920108784567245 . 2586917.