B-B' proteins from small nuclear ribonucleoproteins have an endoribonuclease catalytic domain inactive in native particles

Native small nuclear ribonucleoproteins (snRNPs) purified by several conventional procedures or reconstituted in vitro have no ribonuclease activity. However, when these same snRNPs are centrifuged in cesium chloride gradients at low Mg2+] and in the presence of sarkosyl, an endoribonuclease is unmasked at the density of core particles (i.e. containing only the set of low molecular weight proteins common to all snRNPs), while an inhibitory component is released in soluble form. The nature of this inhibitor was not further investigated and the molecular events underlying this inhibition/activation process remained only a matter of speculation. On the other hand, evidence was obtained that the nuclease activity is carried by B-B' on the basis of its comigration with B-B' as well as with two of their cleavage products after SDS/polyacrylamide gel electrophoresis of snRNP proteins. One was identified by a B-B'-specific monoclonal antibody. Another one, especially prominent and migrating between D and E core proteins, was identified as the N-terminal half of B-B' by microsequence analysis. Although tightly associated with core snRNPs, the activity is not dependent upon the presence of an snRNA. For the time being, the functional significance of this nuclease remains entirely elusive.