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spinqualitylabelsall models Structure of Anaplastic Lymphoma Kinase 7N00 Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Contents 1 Background 2 Structure 2.1 Domains 2.1.1 Three Helix Bundle-like Domain 2.1.2 Poly-Glycine Domain 2.1.3 Tumor-Necrosis Factor-like Domain 2.1.4 Epidermal Growth Factor-like Domain 2.2 Binding Site 2.3 Ligands 2.3.1 ALKAL2 2.3.2 ALKAL1 2.4 Dimerization of Anaplastic Lymphoma Kinase 3 Function 4 Disease and Medical Relevance 4.1 Cancer 4.1.1 Pediatric Neuroblastoma

Background

Anaplastic Lymphoma Kinase (ALK) is a transmembrane receptor and a member of the family of Receptor Tyrosine Kinases (RTKs). RTKs are a family of biomolecules that are primarily responsible for biosignaling pathways such as the insulin signaling pathway. ALK was identified as a novel tyrosine phosphoprotein in 1994 in an analysis of Anaplastic Large-Cell Lymphoma, the protein's namesake. A full analysis and characterization of ALK was completed in 1997, properly identifying it as a RTK, and linking it closely to Leukocyte Tyrosine Kinase (LTK).

Structure

ALK is a close homolog of LTK, and together these two homologues constitute a subgroup within the superfamily of insulin receptors (IR). ALK is composed of 1620 amino acids, with three primary domains, the extracellular domain, the transmembrane domain, and the intracellular domain. The intracellular tyrosine kinase domain ranges from residues 1116-1392, and features the C-terminal end (Figure 1). The transmembrane helical domain (TMH) bridges the gap between the intracellular and extracellular regions from residues 1039-1116. The final section of ALK is the extracellular region, which spans from residues 1025 to 1, and contains 8 domains. Of these 8 domains, two regions of the extracellular region can be found; one containing the ligand-binding site of the protein, and another lesser-known subregion. This lesser-known subregion contains 4 domains from residues 1-626; an N-terminal Region (NTR), two meprin–A-5 protein–receptor protein tyrosine phosphatase μ (MAM), and a low-density lipoprotein receptor class A (LDL) domain sandwiched between the two MAM domains. The presence of an LDL domain sandwiched by two MAM domains is a unique feature that ALK does not share with other RTKs. The purpose behind this unique difference is still unclear. The ligand-binding extracellular subregion is the most well-characterized of the two subregions, containing 4 distinct domains from residues 673-1025; a triple helix bundle (THB) domain, a poly Glycine domain (polyG), a tumor necrosis factor-like (TNF-like) domain, and an epidermal growth factor-like domain (EGF-like). All four domains of this subregion of the extracellular region contribute to ligand-binding ^[1]

Domains

Three Helix Bundle-like Domain

The mainly has a structural function overall as it interacts with the tumor necrosis factor-like domain upon ligand binding. The three helix bundle-like domain's α-helix interacts with the helix α-1' and β strand A-1' on the Tumor Necrosis Factor-like domain. This outermost region of the extracellular ligand-binding domain undergoes rigorous structural reorientation upon ligand binding. THB is primarily involved in the dimerization motif of ALK, which dimerizes upon ligand binding. ^[2]

Poly-Glycine Domain

Located between the three helix bundle-like domain and the tumor necrosis factor-like domain, the has an important structural role. The poly-Glycine domain also has a rare and unique structure of left-handed glycine helices with hexagonal hydrogen bonding shown in Figure 2. These 14 glycine helices are unique to ALK's function among other tyrosine kinases, as these types of structures on the binding domain are not present. These helices are rigid structures, providing a strong anchor for the ligand binding site while the other domains undergo drastic conformational rearrangements.^[2]

Tumor-Necrosis Factor-like Domain

The interacts with the three helix bundle-like domain to begin the conformational changes associated with ligand binding. It is located in approximately the midregion of the extracellular region, bridging the gap between the poly-glycine domain and the epidermal growth factor-like domain. This domain also assists in mediating ligand binding with the epidermal growth factor-like domain. In ligand-binding, as previously stated, this domain interacts heavily with the THB to undergo critical conformation changes necessary for dimerization and ligand recognition. ^[2]

Epidermal Growth Factor-like Domain

The is very malleable and repositioning of this domain is essential for activation of the protein. This domain is able to undergo conformational changes with the ligand bound and when in contact with the tumor necrosis factor-like domain. The interface between the EGF-like and TNF-like domains are primarily hydrophobic residues, which enable their flexibility with regards to one another. The main motifs that are apart of the EGF-like domain are major and minor β-hairpins, which are stabilized by 3 conserved disulfide bridges. ^[2]

Binding Site

This site doesn't start out surrounding the ligand, instead the proximity of the ligand allows conformational changes across the protein. The ligands for ALK both have highly positively charged faces that interact with the TNF-like region, the primary ligand-binding site on the extracellular region^[3]. Salt bridges between the positively charged residues on the ligand and negatively charged residues on the receptor form are formed as the ligand approaches connecting the ligand with the receptor. Three of these salt bridges occur between , , and . These strong ionic interactions allow the drastic conformational changes in the extracellular domain that induce the signaling pathway. ^[2]

Ligands

The extracellular ligands of Anaplastic Lymphoma Kinase are ALKAL2 and ALKAL1.

ALKAL2

(Anaplastic Lymphoma Kinase Ligand 2) is a ligand of ALK as well as LTK located in the extracellular region. The full-length ALKAL2 (dimeric) and ALKAL2-AD (monomeric) can both induce dimerization of ALK ^[2]. Structurally, ALKAL2 has a N-termical variable region and a conserved augmentor domain and tends to aggregate in the cell ^[2]. Overexpression of ALKAL2 has been linked to high-risk neuroblastoma in absence of an ALK mutation ^[4] and could potentially have therapeutic opportunities.

ALKAL1

(Anaplastic Lymphoma Kinase Ligand 1) is a monomeric ligand of ALK, in addition to ALKAL2. Structurally, ALKAL1 and ALKAL2 contain an N-terminal variable region and a conversed C-terminal augmentor domain ^[2]. However, in ALKAL1, this N-terminal variable region is shorter, and shares no similar sequences to ALKAL2. Nevertheless, ALKAL1 shares a 91% sequence similarity with ALKAL2. Both ligands include a three helix bundle domain in their structures, with an extended positively charged surface which is used in ligand binding ^[2].

Dimerization of Anaplastic Lymphoma Kinase

After binding to one of its ligands, Anaplastic Lymphoma Kinase undergoes ^[1]. The dimerization causes trans-phosphorylation of specific tyrosine residues which in turn amplifies the signal. It has been presumed that the phosphorylation cascade activates ALK kinase activity ^[1].

Function

Anaplastic Lymphoma Kinase plays a role in cellular communication and in the normal development and function of the nervous system It is present largely in the developing nervous system of a fetus and newborn, and overtime the expression of ALK dwindles with age. In addition to being expressed heavily in the brain, ALK has been shown to be present in the small intestine, testis, prostate, and colon ^[5].

Disease and Medical Relevance

Cancer

In ALK fusion proteins, the ALK fusion partner may cause dimerization independent of ligand binding, causing oncogenic ALK activation ^[1]. The regulation of ALK dimerization by ALKAL points to clear ways to inhibit ALK activity and may offer new therapeutic strategies in multiple disease settings ^[3]. Studies have shown that approximately 70-80% of all patients who have Anaplastic Large Cell Lymphoma (ALCL) contain the genetic complex of the ALK gene and the nucleolar phosphoprotein B23, also called numatrin (NPM) gene translocation, creating the NPM-ALK complex. This chimeric protein is expressed from the NPM promoter, leading to the overexpression of the ALK catalytic domain. This overexpression of ALK is characteristic of most cancers that are linked to tyrosine kinases, as the overexpression of these proteins leads to uncontrollable growth ^[5].

Pediatric Neuroblastoma

Mutations in Anaplastic Lymphoma Kinase can produce oncogenic activity and are a leading factor in the development of some pediatric neuroblastoma cases^[4]. 8-10% of primary neuroblastoma patients are ALK positive^[4] suggesting that ALK overstimulation is a primary factor in propagating the growth of neuroblastoma. This overstimulation of ALK works in concert with the neural MYC oncogene, and uses the ALKAL2 ligand. Tyrosine kinase inhibitors are proposed to inhibit the growth of further neuroblastoma cells, creating a potential pathway of treatment^[4]