FYB

Two isoforms of FYB with different lengths of 120 and 130 kDa (FYB-120 and FYB-130) exist.[8] The 130kDa version has an extra insertion of 46 amino acids and is preferentially expressed in peripheral T cells.[8] The FYB protein has a variety of binding domains: a non-structured N-terminal region, a proline-rich region, two SH3 domains, a FPPP-motif which binds the ENA/VASP protein family, and other tyrosine-based signaling motifs.[11]

FYB is critical for activation and proliferation of T-helper cells (CD4+) and required for chemokine signal transduction in T-helper cells and cytotoxic T cells (CD8+).[11]

FYB regulates cytokine production in T cells as well as in activated NK cells through the FYN-ADAP axis.[9] In T cells, after TCR stimulation, a unique region of FYB, pYDGI, allows phosphorylation of the protein by FYN.[9] After being phosphorylated, ADAP can bind to Carma1, causing NF-κB translocation into the nucleus and cytokine production.[9]

In mast cells, FYB regulates cell adhesion as well as degranulation.[7] In T cells, FYB allows for cell adhesion and migration through blood vessels through the SLP-76-FYB-SKAP1 complex.[10] After being phosphorylated by FYN, FYB can bind to SLP-76.[7] This binding of FYB and SLP-76 regulates "outside-in signaling" or the transfer of signals from outside the cell to inside the cell by integrin.[10] FYB can also bind to SKAP1, which allows SKAP1 to upregulate integrin activity through interactions with Rap1.[8][10] The bacteria Yersinia can interfere with this pathway in macrophages through the secretion of YopH (Yersinia protein tyrosine phosphatase) into the macrophage, which de-phosphorylates FYB and SKAP1, leading to a decrease in integrin activity that results in an inhibition of adhesion, phagocytosis, and cytotoxicity.[8]

FYB is also an important protein for actin remodeling of immune cells.[11] This is thought to occur through the binding of proteins of the ENA/VASP protein family to the FPPPP-motif of the FYB protein.[11]