Left, washed Ia complexes labeled at C199 were treated with rat liver (RL) SII (phospho-cellulose fraction, 0.5 μ g, lane 1; Bio-Gel SP-5 PW fraction, approximately 1 ng, lane 2) or recombinant human (rhu) SII (Bio-Gel SP-5 PW fraction, approximately 6 ng, lane 3). Cleavage products were isolated and resolved on a 25% polyacrylamide gel. Lane M shows 32P-labeled, chemically synthesized pCpGpUpUpU-pUpU. Right, human SII was expressed in E. coli, purified to near homogeneity, resolved on a SDS gel, and silver-stained. Two protein standards (Bio-Rad pre-stained SDS-polyacrylamide gel electrophoresis standards, low range) are indicated in kilodaltons.