Alpha-sarcin

rRNA endonuclease (EC 3.1.27.10, alpha-sarcin) is an enzyme that catalyses the hydrolysis of the phosphodiester linkage between guanosine and adenosine residues at one specific position in the 28S rRNA of rat ribosomes. This enzyme also acts on bacterial rRNA. rRNA endonuclease (EC 3.1.27.10, alpha-sarcin) is an enzyme that catalyses the hydrolysis of the phosphodiester linkage between guanosine and adenosine residues at one specific position in the 28S rRNA of rat ribosomes. This enzyme also acts on bacterial rRNA. A ribosome-inactivating protein produced by the mold Aspergillus giganteus, alpha-sarcin cleaves the portion of ribosomal RNA that forms the small ribosomal substrate. The high specificity of alpha-sarcin and its efficiency of cleavage are point of study and also account for this protein's very high toxicity level. It is believed that the tyrosine amino acid found along the amino acid sequence of alpha-sarcin allows for the specificity when alpha-sarcin binds to the rRNA. It is the alcohol group found on the tyrosine amino acid that allows for this binding. This was determined in tests that removed the alcohol group, replacing tyrosine with phenylalanine, and the binding affinity was greatly reduced. The region of the DNA that makes alpha-sarcin is highly conserved, along with the corresponding sequence on the targeted ribosome. The corresponding sequence on the targeted ribosome is a centered around a guanine nucleotide located on what is called the 'bulged-G motif'. Alpha-sarcin is remarkable in its cleavage specificity. It interacts with a single bond in the targeted ribosome and breaks it, causing the ribosome to be inactivated. The bond in question is the phosphodiester bond within the sarcin/ricin loop (SRL) of the rRNA. The SRL region of the RNA was named after the alpha-sarcin toxin that targets it. The targeted bond is located within the GAGA tetraloop of the RNA in between a guanine and adenine nucleotide. Other ribotoxins also cleave the RNA of the ribosomes, however there are many more points of cleavage- indicating much less specificity. The specificity of alpha-sarcin is so high, that alpha-sarcin can recognize the SRL segment of the ribosome without the rest of the ribosome present. SRLs fold independently, creating the same structure as when they are in a ribosome. This reaffirms the idea that it is the affinity of alpha-sarcin for this specific region of the ribosome that causes the two to bind and react. The key recognition nucleotide on the ribosome is a guanine nucleotide located six nucleotides upstream from the cleave site (this is the same as the above-mentioned 'g-bulge' region). The conditions that allow for the recognition and cleavage include the salinity of the environment. With increased salt concentration, there is increased competition for the alpha-sarcin to reach the 'G-bulge'. This is due to the electrostatic interactions between the cationic side chains of the amino acids of the alpha-sarcin and the phosphates of the ribosome chain. More salt interferes with these two interactions.