AFFINITY PURIFICATION AND CHARACTERIZATION OF (2'-5')OLIGO(ADENYLATE)-DEPENDENT RNASE FROM MOUSE SPLEEN

Murine (2'-5')A(n)-dependent RNase, a key enzyme of the interferon sys tem, was purified from mouse spleen by affinity chromatography to immo bilized (2'-5')A(n). Since the ribonuclease has high affinity to (2'-5 ')A(n), optimal non-denaturing conditions were obtained to disrupt the (2'-5')A(n)-nuclease complex. Low-pH buffers in the presence of 0.1% Triton X-100 removed almost 80% of the enzyme from the (2'-5')A(n)-aga rose, preserving its (2'-5')A(n) binding activity and RNA cleavage fun ction. Purification was monitored using a classical radiobinding assay , ultraviolet covalent cross-linking method and denaturing-renaturing affinity blotting assay. The purified enzyme was a 160-kDa dimer that migrated with an apparent molecular mass of 78 kDa and was > 80% pure, as assessed by silver-stained SDS gels. Both a 160-kDa dimer and 78-k Da monomer were found in the cellular extract at a 5:1 ratio. Binding of radiolabeled (2'-5')A(n) to (2'-5')A(n)-dependent RNase either in c rude extract or in purified form reached equilibrium by 5 h at 4 degre es C. 2-Mercaptoethanol was required to obtain (2'-5')A(n)-binding act ivity but, interestingly, in the absence of this reducing agent, (2'-5 ')A(n)-binding activity was initiated by preincubation with poly(U), a synthetic substrate of the nuclease. This new mechanistic feature ind icates that interaction of poly(U) with nuclease induced a conformatio nal modification allowing, in a second step, the binding of (2'-5')A(n ). Furthermore, when activated by low amounts of (2'-5')A(n), the elut ed purified enzyme degraded mRNA but there was still degradation in th e absence of (2'- 5')A(n). This suggested a loss of regulatory protein (s) during the purification step. Scatchard analysis showed that the p urified enzyme had a K-d of 106 pM for (2'- 5')A(n), similar to estima tes obtained using crude spleen extracts (K-d 112 pM), indicating that the purified nuclease had almost identical (2'-S')A(n)-binding proper ties to those identified in spleen extracts.