SH2 and SH3 domain-containing adapter molecules serve to link signaling proteins such that signals may be properly propagated. Examples of such adapters are Grb2, Nck and Crk. Grb2 is ubiquitously expressed and is critical for signaling by many receptor tyrosine kinases (RTK) through the Ras/MAPK pathway. its SH3 domains complex mSos, the Ras-specific guanine nucleotide exchange factor, and its SH2 domains interact with RTKs either directly or through additional adapters. We have a new isoform of Grb2, Grb2beta, which is restricted to lymphoid tissues, and is expressed in T-cell lines. Cross-linking of the T-cell receptor (TCR) in these lines induces the formation of signaling protein complexes that include Grb2beta. We have hypothesized that Grb2beta is central to proper TCR signaling. Our proposal seeks to define the signaling context of Grb2beta by identifying its physiological binding partners. To this end we have developed antibodies and fusion proteins to probe extracts for Grb2beta and complexed proteins. We also seek to demonstrate formally a function for Grb2(3 in TCR signaling by creating retrovirally transformed lines expressing dominant-negative mutants of Grb2beta. We hope to disrupt endogenous protein function and abrogate responses normally carried out by T-cell in response to TCR crosslinking. Given the central role for Grb2 in coupling numerous receptors to the Ras/MAPK pathway, the study of Grb2beta is clearly worthwhile for the further understanding of normal immune cell responses, and cancer.