12 S small nuclear ribonucleoprotein-associated acidic-and pyrimidine-specific endoribonuclease from calf thymus and L5178y cells.

12 S ribonucleoprotein (RNP) particles were separated from a 45 S RNP complex (Bachmann, M., Zahn, R. K. and Müller, W. E. G. (1983) J. Biol. Chem. 258, 7033-7040) isolated from calf thymus and L5178y cells. The particles were determined to be associated with an acidic endoribonuclease (pI 4.1; pH optimum 6.2). the enzyme requires Mg2+ and is sensitively inhibited by higher NaCl concentrations. The nuclease specifically degrades poly(U) and poly(C) in an endonucleolytic manner; the end-products are 3'-UMP (85%) and 2',3'-cyclic UMP (12%). Poly(A) strongly inhibits the pI 4.1 endoribonuclease activity. The Michaelis constant (for poly(U)) was determined as 82 microM and the maximal reaction velocity was 0.54 mumol/microgram per h. The endoribonuclease is distinguished from the known pyrimidine-specific ribonucleases (pancreatic ribonuclease and endoribonuclease VII) by further criteria, e.g., resistance to thiol reagents, inhibition by EDTA, Mg2+ requirement, pI and pH optimum. Using the techniques of counterimmunoelectrophoresis and immunoaffinity column chromatography it was shown that the pI 4.1 endoribonuclease-associated 12 S RNP particles display antigenicity to anti-Sm and anti-(U1)-RNP antibodies. An RNA component, isolated from the 12 S-45 S hypercomplex, was identified as U1-snRNA.