A novel S3S-TAP-tag for the isolation of T-cell interaction partners of adhesion and degranulation promoting adaptor protein

The identification of modular units of cellular function is a major goal for proteomic research. Protein complexes represent important building blocks defining functionality and deciphering their composition remains a major challenge. Here, we have designed a new tandem affinity purification (TAP) tag (termed S3S-tag) for the isolation of protein complexes. Specifically, the immune cell protein ADAP that regulates integrin adhesion was fused either C- or N-terminally to the S3S-tag. After retroviral transduction of a vector containing S3S-tagged ADAP and internal ribosomal entry site encoded enhanced green fluorescent protein (eGFP), Jurkat T cells were sorted according to eGFP expression and further selected for expression of TAP-tagged protein close to endogenous levels. The combination of a cleavable S-tag and a Strep-tag II allowed for the isolation of ADAP and associated proteins. Subsequently, stable isotope labeling with amino acids in cell culture-based mass spectrometric analysis was performed to identify potentially specific interaction partners. Co-purification of the known interaction partner Src kinase-associated phosphoprotein of 55 kDa indicates the validity of our approach, while the identification of the ENA/VASP family member EVL, the guanine nucleotide exchange factor GEF-H1 and the adaptor protein DOCK2 corroborates a link between ADAP-mediated integrin regulation and the cytoskeleton.