The proteins of the complement system participates in the regulation of various phases of the immune response. Complement activation products bind to immune complexes and form multivalent complement receptor 2 (CR2) ligands that regulate B cell proliferation and differentiation. The potential for using CR2 ligands to enhance antibody production urges the deciphering of the subcellular mechanisms that are involved in the regulation of transcription of the CR2 gene and the expression of its product on the surface membrane. We have identified a heterogeneous ribonucleoprotein (hnRNP) which binds to a previously unidentified DNA motif that is present in the promoter region of CR2 and is involved in the regulation of the transcription of the CR2 gene. On the basis of these findings we hypothesize that the hnRNP undergoes modification (phosphorylation) in response to external stimuli, binds to a DNA motif and acts in concert with other DNA binding proteins to control the transcription of the CR2 gene. The overall goal of this proposal is to complete the structural and functional characterization of the hnRNP protein and characterize the mechanisms that are involved in the regulation of the expression of complement CR2. Specifically we intend to structurally and functionally characterize the hnRNP that binds a CR2-promoter region-defined oligonucleotide and increases its transcriptional activity. This will be accomplished by determining the primary structure of hnRNP, by structurally and functionally analyzing the CR2-promoter region defined oligonucleotide binding and other functional domains of the hnRNP. We will characterize the mechanisms whereby external stimuli regulate the expression of CR2. This will be accomplished by investigating the effect of lymphokines on the transcription (CR2-promoter region defined oligonucleotide-driven reporter gene construct) and expression of the CR2 gene (mRNA and surface protein) and by characterizing the interactions of CR2 promoter region binding proteins. Since we have found that sera from patients with systemic autoimmune diseases have antibodies directed against this hnRNP, we propose to study their frequency and clinical significance.