The human AXL gene is located on chromosome 19q13.2 and encodes 20 exons. Exons 1-10 encode the extracellular domain, which includes a signal peptide (exon 1), two immunoglobulin (Ig) domains (exons 2-3 and 4-5), and two fibronectin type III (FNIII) domains (exons 6-7 and 8-9). Exon 11 encodes a short extracellular region subject to proteolytic cleavage (see protein description), as well as the entire transmembrane domain. Exons 12-20 encode the intracellular domain, which includes the tyrosine kinase domain (exons 13-20) (OBryan et al., 1991; Hubbard et al., 2009).

There are two 4.7 kb mRNA variants of AXL distinguished by the presence or absence of exon 10, a 27 bp region in the C-terminal end of the extracellular domain, via alternative splicing. Both variants exist ubiquitously and at much higher levels in many cancers. Although the longer transcript is more highly expressed in tumor tissue relative to its shorter counterpart, both forms of the protein have the same transforming potential (OBryan et al., 1991).

The full-length AXL protein contains 894 amino acids and has a molecular weight of 104 kDa. As the extracellular domain contains six N-linked glycosylation sites, two other post-translationally modified forms weighing 120 and 140 kDa -representing partial and complete glycosylation, respectively- have been identified. The extracellular component of the AXL receptor contains two Ig-like domains (aa 37-124 for domain 1, 141-212 for domain 2) followed by two FNIII domains (aa 224-322 for domain 1, 325-428 for domain 2) (OBryan et al., 1991). This particular tandem arrangement defines AXL as part of the TAM family of receptor tyrosine kinases (RTKs), which also includes TYRO3 and MERTK (Graham et al., 1994). All three TAM family proteins bind the ligand GAS6, a vitamin K-dependent protein structurally similar to Protein S (PROS1), which activates MERTK and TYRO3 but not AXL (Prasad et al., 2006). Like all TAM family members, each immunoglobulin domain of the AXL receptor provides a binding site for each of the two laminin G-like (LG) domains of GAS6, the only identified ligand for AXL as of yet (Sasaki et al., 2006).
Carboxy-terminal to the second FNIII domain, fourteen amino acids (aa 438-451 in the longer variant) serve as a proteolytic cleavage site, yielding an 80 kD soluble form of AXL with only the extracellular domains of the full-length protein. As this cleavage site translates from exon 11, proteins from both transcript variants are subject to proteolysis. The intact ligand-binding domains in this soluble form highlight its potential role in signal transduction as an inhibitor of the membrane-bound receptor (OBryan et al., 1995).
The intracellular tyrosine kinase domain of AXL contains the sequence KW(I/L)A(I/L)ES (aa 714-720), which is conserved among all TAM family RTKs. Within this signature motif, the third and fifth amino acids are isoleucine (I) in both AXL and MERTK, while leucine (L) occupies these positions in TYRO3 (Graham et al., 1994). Activation of the AXL receptor occurs within its intracellular domain and is characterized by the phosphorylation of tyrosine residues at sites that have yet to be defined. MERTK is the only TAM family member with validated tyrosine autophosphorylation sites; AXL also has three tyrosine residues -Y697, Y702, and Y703- conserved in sequence context within its kinase domain, but no evidence exists implicating their role in autophosphorylation (Ling et al., 1996). Numerous mass spectrometry analyses confirm that these and several other tyrosine residues are, in fact, phosphorylated (Hornbeck et al., 2004), and a recent study demonstrated that phosphorylation occurs at Y702 and Y703 upon GAS6 stimulation (Pao-Chun et al., 2009). However, neither of these sites has been shown to directly regulate or interact with the downstream effectors of AXL activation.
Three other tyrosine residues within the AXL intracellular domain -Y779, Y821, and Y866- mediate binding of various substrates, suggesting that they may be more likely candidates for autophosphorylation sites. Y779 partially contributes to binding PI3K, while Y866 plays an integral role in binding PLC. Y821 has been shown to be a critical docking site for multiple substrates, including PI3K, PLC, GRB2, c-SRC, and LCK (Braunger et al., 1997). Despite this evidence, an in vivo study refuted the significance of Y821 in AXL autophosphorylation and activation, as mutants without Y821 display normal GAS6-stimulated tyrosine phosphorylation (Fridell et al., 1996).
Along with conventional ligand-induced dimerization and autophosphorylation, AXL activation can also occur through ligand-independent pathways. AXL overexpression causes homophilic binding between its extracellular domains on neighboring cells and leads to increased phosphorylation of its intracellular domain (Bellosta et al., 1995). AXL also engages in cross-talk with the IL-15 receptor, which transactivates AXL and requires it for survival from TNF-alpha-mediated apoptosis (Budagian et al., 2005).

AXL is expressed throughout all tissue and cell types (OBryan et al., 1991). Higher expression is observed in endothelial cells, heart and skeletal muscle, liver, kidney, testis, platelets, myelomonocytic cells, hippocampus, and cerebellum (Neubauer et al., 1994; Bellosta et al., 1995; Graham et al., 1995; Angelillo-Scherrer et al., 2001). Relative to normal expression levels, AXL is increased in a number of disease states as reviewed by Linger et al (2008).

AXL is a transmembrane receptor tyrosine kinase.

Activation of the AXL receptor initiates various signaling pathways involved in cell survival, proliferation, apoptosis inhibition, migration, cell adhesion, and cytokine production. This is mediated via interactions with a spectrum of signaling molecules, including PI3K/Akt, ERK1/ERK2, GRB2, RAS, RAF1, MEK-1, and SOCS-1. Beyond its overexpression and oncogenic potential in numerous cancers, AXL has also been implicated in angiogenesis and metastasis (Linger et al., 2008).

AXL and the two other TAM family members, MERTK and TYRO3, share 31-36% and 54-59% sequence identities in the extracellular and intracellular regions, respectively (Graham et al., 1995).

Although AXL overexpression is implicated in oncogenesis, no mutations in the gene have been identified as the underlying cause.