The site of action of alpha-sarcin on eukaryotic ribosomes. The sequence at the alpha-sarcin cleavage site in 28 S ribosomal ribonucleic acid

Abstract

alpha-Sarcin is a cytotoxic protein that inhibits eukaryotic elongation factor 1 catalyzed binding of aminoacyl-tRNA to eukaryotic ribosomes as a result of cleaving a fragment from the large RNA in the 60 S ribosomal subunit. Rat liver polysomes are sensitive to alpha-sarcin in vitro only if the treatment is carried out in the presence of EDTA. The oligonucleotide produced by alpha-sarcin action on rat liver ribosomes is derived from the 3' end of 28 S rRNA and has a molecular weight of approximately 161,000 (about 488 bases). The sequence at the site of cleavage (the 5' end of the oligonucleotide produced by alpha-sarcin action on rat liver ribosomes) was AGGAAC for both rat liver 28 S rRNA and yeast 25 S rRNA. Based on the size of the alpha-fragment derived from Escherichia coli ribosomes (about 243 nucleotides), it is likely that a similar sequence, AGGACC, occurs at the site of cleavage of prokaryotic 23 S rRNA. It is also likely that the sequence on the 5' side of the alpha-sarcin cleavage site is UAGUACGAG. The sequence UAGUACGAGAGGAAC must be important because it is conserved and because hydrolysis of a single phosphodiester bond there inactivates the ribosome. We suggest that this region of the large nucleic acid in the 60 S subparticle is involved in binding aminoacyl-tRNA to the ribosome, first, because it is the eukaryotic elongation factor 1 catalyzed reaction that is inhibited by alpha-sarcin, and second, because peptidyl-tRNA seems to protect polysomes against the toxin. The function of EDTA appears to be to remove peptidyl-tRNA since puromycin also sensitizes polysomes to alpha-sarcin.