We recently completed the first comprehensive study of tyrosine phosphorylated substrates in resting and BCR-stimulated primary B cells. From this effort, we focused on a novel transmembrane adaptor protein, termed Twixt, due to its novelty, selective expression in B cells and potential importance in fulfilling a knowledge gap in our understanding of BCR signaling - namely how BLNK is inducibly recruited to the BCR complex. In the proposed work, we will determine how Twixt is recruited and utilized by the BCR and perhaps other receptors to effect downstream signaling. These data will provide a mechanistic framework for interpreting the phenotypes observed in Twixt-deficient mice, which will be generated and analyzed in the second Aim. Completion of the proposed studies will establish the prominence of Twixt as a central player in BCR signaling, or reveal a more selective role. PUBLIC HEALTH RELEVANCE: Signaling via the B cell antigen receptor (BCR) determines the fate of the B cell in terms of continued survival, proliferation and differentiation. These signals are relayed to the nucleus to bring about changes in gene expression that will decide B cell fate. The nature of these signals remains incomplete - as is our understanding of how aberrant signal transduction can precipitate B cell autoimmunity or transformation. In the proposed work we will characterize a novel B cell signaling adaptor that may be quite important in parsing this early membrane-proximal signal to engage downstream effector pathways.