Tyrosine-protein kinase that acts downstream of cell surface receptors for growth factors and plays a role in the regulation of the actin cytoskeleton, microtubule assembly, lamellipodia formation, cell adhesion, cell migration and chemotaxis. Acts downstream of EGFR, KIT, PDGFRA and PDGFRB. Acts downstream of EGFR to promote activation of NF-kappa-B and cell proliferation. May play a role in the regulation of the mitotic cell cycle. Plays a role in the insulin receptor signaling pathway and in activation of phosphatidylinositol 3-kinase. Acts downstream of the activated FCER1 receptor and plays a role in FCER1 (high affinity immunoglobulin epsilon receptor)-mediated signaling in mast cells. Plays a role in the regulation of mast cell degranulation. Plays a role in leukocyte recruitment and diapedesis in response to bacterial lipopolysaccharide (LPS). Phosphorylates CTTN, CTNND1, PTK2/FAK1, GAB1, PECAM1 and PTPN11. May phosphorylate JUP and PTPN1. Can phosphorylate STAT3 according to PubMed:10878010 and PubMed:19159681, but clearly plays a redundant role in STAT3 phosphorylation. According to PubMed:11134346, cells where wild type FER has been replaced by a kinase-dead mutant show no reduction in STAT3 phosphorylation. Phosphorylates TMF1. Isoform 3 lacks kinase activity. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Activated by phosphatidic acid binding (By similarity). Activated by hydrogen peroxide (in vitro). Activated by reactive oxygen species (ROS). Homotrimer. Isoform 4 is a monomer, due to the absence of the N-terminal coiled coil domains. Interacts with CTNND1, EGFR, FLT3, PECAM1 and PDGFR. Interacts (via SH2 domain) with CTTN. Component of a complex that contains at least FER, CTTN and PTK2/FAK1 (By similarity). Interacts with IRS1 and PIK3R1. Interacts with STAT3. Interacts with PPP1CA and regulates its phosphorylation at 'Thr-320'. Interacts with JAK1. Interacts with HSP90; this stabilizes phosphorylated FER and protects FER against proteasomal degradation. Interacts with ARHGDIA, NRP1, PLEC and TMF1. Detected on microtubules in polarized and motile vascular endothelial cells. Colocalizes with F-actin at the cell cortex. Colocalizes with PECAM1 and CTNND1 at nascent cell-cell contacts (By similarity). Not detected in the nucleus, but detected in the nuclear area surrounding the chromosomes after breakdown of the nuclear envelope during mitosis (PubMed:11339827). Detected in liver and testis. Isoform 4 is detected only in testis (at protein level). Widely expressed. Up-regulated by insulin in myogenic cells (in vitro). The coiled coil domains mediate homooligomerization and are required for location at microtubules. The N-terminal region including the first coiled coil domain mediates interaction with phosphoinositide-containing membranes. Autophosphorylated. Polyubiquitinated; this leads to proteasomal degradation. No visible phenotype, and the mice are fertile. Mice have reduced CTTN phosphorylation. Mice lacking both Fps/Fes and Fer activity are viable and fertile, but produce slightly fewer pups per litter than normal. They display elevated levels of circulating neutrophils, erythrocytes and platelets, while other cell counts are normal. Belongs to the protein kinase superfamily. Tyr protein kinase family. Fes/fps subfamily.