Association of a polyuridylate‐specific endoribonuclease with small nuclear ribonucleo‐proteins which had been isolated by affinity chromatography using antibodies from a patient with systemic lupus erythematosus

Immunoglobulins, containing antibodies against U1-snRNP, have been prepared from a patient with systemic lupus erythematosus. After coupling these antibodies to a Sepharose matrix, U-snRNPs have been isolated and purified from rat liver nuclei by use of immunoaffinity chromatography. The resulting RNPs had the typical protein pattern of U-sn RNPs and a sedimentation coefficient of 12 S. The U-snRNP preparation was associated with an endoribonuclease which required Mg2+ for optimal activity. The enzyme, with an pH optimum of 6.2, degraded only poly(U). Other single-stranded polyribo- and polydeoxyribonucleotides, tRNA, as well as double-stranded RNA and DNA were not digested. The products of a terminal digestion are (U)6-12 with 3'-OH and 5'-P termini. The possible involvement of this endoribonuclease in the splicing of hnRNA is discussed.