Stoichiometry of 2',5'-oligoadenylate-induced dimerization of ribonuclease L. A sedimentation equilibrium study

Ribonuclease L is an endoribonuclease that is activated by binding of 2',5'-linked oligoadenylates. Activation of ribonuclease L also induces dimerization. Here, we demonstrate using equilibrium sedimentation that dimerization requires the binding of one 5'-monophosphate 2',5'-(adenosine)3 molecule per ribonuclease L monomer. No dimerization was observed in the absence of activator up to a protein concentration of 18 microM, indicating that unliganded enzyme is unable to dimerize or the association is very weak. In parallel with dimerization, enzymatic activity is also maximized at a 1:1 activator: ribonuclease L stoichiometry. The same stoichiometry for dimerization is observed using a nonphosphorylated activator 2'-5'-(adenosine)3. Adenosine triphosphate or RNA oligonucleotide substrates do not induce dimerization. The observed stoichiometry supports a model for ribonuclease L dimerization in which activator binds to monomer, which subsequently dimerizes.