Background

The anaplastic lymphoma kinase (ALK) was first discovered in 1994 as a tyrosine kinase in anaplastic large-cell lymphoma (ALCL) cells.^[1] The specific type of tyrosine kinase ALK is classified as is a receptor tyrosine kinase (RTK) and like other RTKs, it's an integral protein with extracellular and intracellular domains and is involved in transmembrane signaling and communication within the cell. ALK is commonly expressed in the development of the nervous system. Anaplastic lymphoma kinase receptor (ALKr) is the extracellular portion of the RTK that includes a binding surface for a ligand to bind. When the ALK activating ligand (ALKAL) binds to ALKr, this causes a conformational change of ALK, allowing two ALK-ALKAL complexes to interact with each other, which will then allow intracellular kinase domain of ALK to phosphorylate a tyrosine residue on a downstream enzyme, which will activate this enzyme and activate a signaling cascade. Abnormal forms of ALK are closely related to the formation of several cancers. ^[2]

Structure & Function

Domains

Figure 1: Anaplastic Lymphoma Kinase and its domains.

ALKr is in its inactive state as a and has many different domains (Figure 1) that are important to the formation of the active state, leading to ALK's main function. The tumor necrosis factor-like domain (TNFL), glycine-rich domain (GlyR), polyglycine extension loop (PXL), and growth factor-like domain (EGF) are the main domains of ALKr, and the only domains whose structures have been fully discovered are in color (Figure 1). The (cyan) is the domain that binds to the TMH (transmembrane region), connecting the extracellular portion of ALK to the intracellular kinase domain. (orange) has a beta-sandwich structure that provides important residues that act as the binding surface for the ligand. (green) contains 14 rare polyglycine helices that are hydrogen-bound to each other. The of these rare helices create a rigid structure which allows it to function as a scaffold to anchor the ligand-binding site on the TNF-like domain while bound to the ligand. The numerous hydrogen bonds are what create this of the helices and the rigidity of the structure. The (pink) connects two of these polyglycine helices, and it also plays a role in forming important interactions of the dimerized activated state of ALKr. Additional domains are present in ALK monomers (Figure 1), but their structures are not currently known. The heparin binding domains (HBDs), are at the N-terminal end of the monomer. Heparin is a likely activating ligand of ALK.^[3] The transmembrane domain (TMH) contains the membrane spanning portion of ALK that transmits extracellular ligand binding into an intracellular signal. The kinase domain is the intracellular portion of ALK that contains the Tyr residues which are auto-phosphorylated when ALK is activated, initiating a signaling cascade. ^[4]

Figure 2: ALK-ALKAL complex, showing the conformation change of ALK from the binding of ALKAL. PDB: 7N00

Membrane Guidance of ALKAL to ALK

The first step to the activation of ALK is to bind the ALK activating ligand (ALKAL) to ALKr. is a triple alpha-helix polypeptide structure that signals for a conformational change of ALK. The cell membrane allows for the interaction between the ALKAL and ALKr. The negatively charged phosphate groups on the cell membrane interact with a highly conserved positively charged on ALKAL that faces the membrane. These (Lys96, His99, Lys100) guide ALKAL to ALKr and correctly positions ALKAL for its binding surface to face ALKr's ligand site, which allows for a more favorable interaction. This interaction causes a conformational change, forming the .

Conformational Change

ALKAL to ALKr at the TNFL domain, which has important negatively charged residues that form with positively charged residues on ALKAL. These bonds initiate the conformational change, as these residues can only come into close proximity with each other if the conformational change occurs. The PXL and GlyR domains hinge forward when the change is initiated^[5] (Figure 2). Glu978, Glu974, Glu859, and Tyr966 are the residues of ALKr that form these bonds with Arg123, Arg133, Arg136, Arg140, and Arg117 of ALKAL. Once the ALKr-ALKAL complex is formed, the of two ALKr-ALKAL complexes occurs. The main driving force of the interaction between two ALKr-ALKAL complexes that become a dimer are hydrophobic interactions of the PXL loop of one ALKr with the other complex's ALKAL and TNFL domain of ALKr. This dimer of two ALKr-ALKAL complexes is the active form of ALK and is now able to perform its main function of phosphorylation.

Role and Function of Dimerized ALKr-ALKAL

The value of the ALKr-ALKAL dimer is its rigidity and strength of its structure, being very important for the function of ALK. Each step of building from the ALK monomer to the dimer adds a level of strength in the structure, making it more likely to create the conformational change the kinase domain undergoes inside the cell across the membrane. This conformation into the dimer initiates a conformational change of the intracellular kinase domain of ALK. This causes an autophosphorylation of several tyrosine residues of this domain, which ultimately activates the kinase function of ALK and can now phosphorylate another protein or enzyme downstream in the signaling cascade. An example of a kinase that has a similar function to ALK are insulin receptors (IR). Their ligand (insulin) initiates a conformational change, which allows the kinase domain to be autophosphorylated, activating IR and allowing it to activate other enzymes or proteins down multiple possible signaling pathways via phosphorylation.

Disease

Many have been identified that cause constitutive receptor activation. Enhanced receptor interaction or stabilization of active receptors also increase oncogenic potentials (Figure 3). One gain-of-function mutation found in lung adenocarcinoma is that is mutated to arginine and leads to constitutive activation of ALK (Fig. 3b). What causes this gain-of-function is the increased length of arginine compared to histidine, which allows it to interact with the other ALKr-ALKAL complex via ionic bonding. This creates a stronger interaction between the two complexes, which increases the likelihood for ALK to be activated at any given time, which then also keeps the whole signal pathway activated downstream. Mutation of to arginine could cause possible oncogenic potentials which are not specified yet (Fig. 3a), but this mutation would have a similar effect as His694Arg because it is an even bigger increase in length compared to the hydrogen side group of glycine. It is also a change from a nonpolar to a polar/charged side group, which will create ionic bonds with the other ALKr-ALKAL complex, making it an even bigger increase of interaction between the two complexes. The F856S and R753Q are two other mutations in ALK that are known to increase cytokine-dependent cell proliferation in certain cells. ^[6]

Figure 3: Mutated residues on ALK that contribute to stabilization of the active state of ALK, leading to many types of cancers. A: Gly747Arg. B: His694Arg. PDB: 7N00