Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by cutaneous manifestations and the production of autoantibodies. Recent studies have demonstrated that the U1-70 kD component of the U1-small nuclear ribonucleoprotein particle (U1-snRNP) and other autoantigens undergo proteolytic cleavage by caspases during apoptosis, an event that correlates with the relocalization of the U1-snRNP to cell surface "apoptotic blebs" in cultured keratinocytes. It is hypothesized that proteolytic cleavage and other posttranslational modifications target autoantigens to apoptotic blebs, allowing autoantigens to bypass or overcome normal mechanisms of tolerance, ultimately leading to autoantibody production. We recently described 8 autoantigens (termed pp2OO, pp9O, pp54, pp46, pp42, pp34, pp23, and pp17) that are phosphorylated in Jurkat T cells undergoing apoptosis. Four of these proteins (i.e., pp54, pp42, pp34, and pp23) associate with the U1-snRNP autoantigen complex during apoptosis and have been identified as serine/arginine-rich splicing factors (SR proteins), which comprise a structurally-related family of important regulatory splicing factors. We hypothesize that the association of phosphorylated SR proteins with the U1-snRNP splicing complex during apoptosis leads to the localization of the phospho-SR/U1-snRNP complex in apoptotic blebs where they are perfectly situated to interact with antigen presenting cells, or directly with B and T cells. The broad, long-term goal of this proposal is to identify whether the phosphorylation of SR proteins in apoptotic keratinocytes is associated with localization to cell surface blebs and the subsequent development of autoantibodies to the phospho-SR protein/U1-snRNP complex in patients with SLE and in a murine model of lupus (the MRL/lpr mouse). There are three specific aims of this proposal: i.) To determine if lpr mice mount a B cell response to phosphorylated SR proteins; ii.) To determine whether the U1-snRNP complex is associated with phosphorylated SR proteins in apoptotic blebs of keratinocytes derived from MRL/lpr mice; and iii.) To determine whether phosphorylated SR proteins coassociate with the U1-snRNP in apoptotic blebs of ultraviolet (UV) -irradiated keratinocytes derived from healthy control patients or patients with cutaneous SLE. The results of this proposal may prove useful in identifying pathogenic mechanisms underlying illnesses associated with skin disease and the production of autoantibodies, including SLE, Sjogren's disease and scleroderma.