AN AUTOANTIBODY DERIVED FROM MICE WITH EXPERIMENTAL SYSTEMIC LUPUS-ERYTHEMATOSUS IS DIRECTED AGAINST THE ESSENTIAL SPLICING FACTOR SF53 4 -A POSSIBLE ROLE FOR LARGE NUCLEAR RIBONUCLEOPROTEIN-PARTICLES IN AUTOIMMUNE DISORDERS/

We have previously shown that nuclear transcripts of several pre-mRNAs can be released from nuclei of mammalian cells in a form of large nuc lear ribonucleoprotein (InRNP) particles. These particles, which invar iably sediment at the 200S region in sucrose gradient, contain all U s mall nuclear RNPs required for pre-mRNA splicing and a multitude of he terogeneous nuclear RNP proteins. From a panel of mAbs raised against the InRNP particles, a specific mAb (53/4) identified a nuclear protei n of 88 kDa as an essential splicing factor (SF53/4). In a parallel in dependent study, mAbs were established in mice with experimental syste mic lupus erythematosus (SLE), that had been induced by immunization w ith a murine mAb against a human anti-DNA mAb bearing the common 16/6 idiotype. One of the produced mAbs (2C5/3) recognized an 88 kDa RNP pr otein. In the present study we have used the following criteria to dem onstrate that mAb 2C5/3 and mAb 53/4 recognize the same protein. First , mAb 2C5/3 inhibited splicing by direct addition. Second, the 88 kDa polypeptide that had been immunodepleted from HeLa cells nuclear extra ct by mAb 2C5/3 was recognized by mAb 53/4 in protein blots. Third, th e HeLa nuclear extract depleted by mAb 2C5/3 was devoid of splicing ac tivity and could not assemble into splicing complexes with exogenous p re-mRNA; however, splicing and spliceosome assembly activities were re stored to such a defective extract by adding back the 88 kDa protein t hat had been purified by immunoaffinity binding to immobilized mAb 53/ 4. These results demonstrate that mAb 2C5/3, which was derived from mi ce with experimental SLE, is directed against epitope(s) involved in t he activity of SF53/4. The fact that SF53/4 is an integral component o f the InRNP particles implies the possibility that such particles are involved in eliciting an autoimmune response.