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Anaplastic Lymphoma Kinase (ALK) is a [https://en.wikipedia.org/wiki/Transmembrane_protein transmembrane] receptor and a member of the family of [https://proteopedia.org/wiki/index.php/Receptor_tyrosine_kinases 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 [https://lymphoma.org/aboutlymphoma/nhl/alcl/ Anaplastic Large-Cell Lymphoma], the protein's namesake.<ref name ="Huang" /> A full analysis and characterization of ALK was completed in 1997, properly identifying it as a RTK, and linking it closely to [https://en.wikipedia.org/wiki/Leukocyte_receptor_tyrosine_kinase Leukocyte Tyrosine Kinase] (LTK).<ref name ="Huang" /> ALK's normal activity as a receptor tyrosine kinase is to transfer a gamma-phosphate group from adenosine triphosphate (ATP) to a tyrosine residue on it's substrate.<ref name ="Huang" /> ALK is one of more than 50 RTKs encoded within the human genome, <ref name ="Huang" /> and it's tyrosine kinase activity seems to be especially important in the developing nervous system. <ref name ="Huang" /> ALK is most commonly associated with oncogenesis, as various factors, including overstimulation, lead to extreme cell proliferation.

ALK is a close homolog of LTK, and together these two homologues constitute a subgroup within the superfamily of [https://proteopedia.org/wiki/index.php/Insulin_receptor insulin receptors] (IR). ALK is composed of three primary regions: the extracellular region, the transmembrane region, and the intracellular region. [[Image:Full ALK Structure Graphic.PNG|600 px|right|thumb|Figure 1. Overview of Anaplastic Lymphoma Kinase Structure with domains where known structure are color coordinated and other domains are grayed out.]] The extracellular region of ALK contains 8 total domains within 2 fragments. A Three Helix Bundle-like domain (THB-like), a Poly-Glycine domain (GlyR), a Tumor Necrosis Factor-like domain (TNF-like), and an Epidermal Growth Factor-like domain (EGF-like) make up the ligand binding fragment while a N-terminal domain, two [https://en.wikipedia.org/wiki/Meprin_A meprin–A-5] protein–receptor protein tyrosine phosphatase μ (MAM) domains and a [https://en.wikipedia.org/wiki/Low-density_lipoprotein low-density lipoprotein] receptor class A (LDL) domain sandwiched between the two MAM domains make up the second fragment. All four domains of the ligand binding fragment of the extracellular region contribute to ligand-binding <ref name ="Huang" />. 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 [https://en.wikipedia.org/wiki/Transmembrane_domain transmembrane helical region] (TMH) bridges the gap between the intracellular and extracellular regions. The intracellular tyrosine kinase domain features the Kinase domain and the C-terminal end (Figure 1).  

=== Extracellular Domains ===

The <scene name='90/904332/Thb-like_domain/1'>Three Helix Bundle-like Domain</scene> mainly has a structural function as it interacts with the TNF-like domain upon ligand binding.<ref name="Reshetnyak" /> The THB-like domain's α-helix interacts with the helix α-1' and β strand A-1' on the TNF-like domain.<ref name="Reshetnyak" /> This outermost region of the extracellular ligand-binding domain undergoes rigorous structural reorientation upon ligand binding.<ref name="Reshetnyak" /> The THB-like is primarily involved in the dimerization motif of ALK, which dimerizes upon ligand binding. <ref name="Reshetnyak" />

[[Image:glycinehelicesorange.png|300 px|right|thumb|Figure 2. Rare Glycine helices on Anaplastic Lymphoma Kinase; The structure of extracellular ALK is shown in a perpendicular, cross sectional way, highlighted in orange.In black, hydrogen bonds are structured in a hexagonal-like way.]]Located between the THB-like domain and the TNF-like domain, the <scene name='90/904331/Polyg_region1/4'>Poly-Glycine Region</scene> has an important structural role.<ref name="Reshetnyak" /> The GlyR domain also has a rare and unique structure of left-handed glycine helices with hexagonal hydrogen bonding shown in Figure 2.<ref name="Reshetnyak" /> 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.<ref name="Reshetnyak" /> These helices are rigid structures, providing a strong anchor for the ligand binding site while the other domains undergo drastic conformational rearrangements.<ref name="Reshetnyak" />

The <scene name='90/904331/Tnf-like_domain/2'>Tumor Necrosis Factor-like Domain</scene> interacts with the THB-like domain to begin the conformational changes associated with ligand binding.<ref name="Reshetnyak" /> It is located in approximately the midregion of the extracellular region, bridging the gap between the GlyR domain and the EGF-like domain. This domain also assists in mediating ligand binding with the EGF-like domain.<ref name="Reshetnyak" /> In ligand-binding, as previously stated, this domain interacts heavily with the THB-like domain to undergo critical conformation changes necessary for dimerization and ligand recognition. <ref name="Reshetnyak" />

The <scene name='90/904331/Egf_like_domain/3'>Epidermal Growth Factor-like Domain</scene> is very malleable and repositioning of this domain is essential for activation of the protein.<ref name="Reshetnyak" /> This domain is able to undergo conformational changes with the ligand bound and when in contact with the TNF-like domain.<ref name="Reshetnyak" /> The interface between the EGF-like and TNF-like domains are primarily hydrophobic residues, which enable their flexibility with regards to one another.<ref name="Reshetnyak" /> 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. <ref name="Reshetnyak" />

The extracellular ligands of ALK are ALKAL2 and ALKAL1.  

<scene name='90/904331/Alkal2/3'>ALKAL2</scene> (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 <ref name="Reshetnyak">PMID:34819673</ref>. Structurally, ALKAL2 has a N-termical variable region and a conserved augmentor domain and tends to aggregate in the cell <ref name="Reshetnyak" />. Overexpression of ALKAL2 has been linked to high-risk [https://en.wikipedia.org/wiki/Neuroblastoma neuroblastoma] in absence of an ALK mutation <ref name="Borenas">PMID:33411331</ref> and could potentially have therapeutic opportunities.  

<scene name='90/904331/Alkal1/5'>ALKAL1</scene> (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 <ref name="Reshetnyak" />. 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 <ref name="Reshetnyak" />.

This site doesn't start out surrounding the [https://en.wikipedia.org/wiki/Ligand_(biochemistry) ligand], instead the proximity of the ligand allows [https://en.wikipedia.org/wiki/Conformational_change 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<ref name="Li" />. [https://en.wikipedia.org/wiki/Salt_bridge_(protein_and_supramolecular) 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 <scene name='90/904331/Salt_bridge_overview/1'>salt bridges</scene> occur between <scene name='90/904331/Salt_bridge_859_140/3'>E859 and R140</scene>, <scene name='90/904331/Salt_bridge_974_136/4'>E974 and R136</scene>, and <scene name='90/904331/Salt_bridge_978_123_133/3'>E978 with both R123 and R133</scene>. These strong ionic interactions allow the drastic conformational changes in the extracellular domain that induce the signaling pathway. <ref name="Reshetnyak" />

After binding to one of its ligands, ALK undergoes <scene name='90/904331/Alk_full_dimerization/3'>ligand-induced dimerization</scene> <ref name="Huang">PMID:30400214</ref>. The [https://en.wikipedia.org/wiki/Dimer_(chemistry) dimerization] causes trans-phosphorylation of specific [https://en.wikipedia.org/wiki/Tyrosine tyrosine] residues which in turn amplifies the signal. It has been presumed that the [https://en.wikipedia.org/wiki/Phosphorylation_cascade phosphorylation cascade] activates ALK kinase activity <ref name="Huang" />.

ALK plays a role in [https://en.wikipedia.org/wiki/Cell_signaling cellular communication] and in the normal development and function of the [https://en.wikipedia.org/wiki/Nervous_system nervous system]<ref name ="Huang" /> ALK is present largely in the developing nervous system of a fetus and newborn, and overtime the expression of ALK dwindles with age.<ref name ="Huang" /> In addition to being expressed heavily in the brain, ALK has been shown to be present in the small intestine, testis, prostate, and colon <ref name="Della Corte">PMID:29455642</ref>.  

In ALK [https://en.wikipedia.org/wiki/Fusion_protein fusion proteins], the ALK fusion partner may cause dimerization independent of ligand binding, causing oncogenic ALK activation <ref name="Huang" />.  

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. This complex 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 <ref name="Della Corte">PMID:29455642</ref>.

Mutations in ALK can produce oncogenic activity and are a leading factor in the development of some pediatric neuroblastoma cases<ref name="Borenas" />. 8-10% of primary neuroblastoma patients are ALK positive<ref name="Borenas" /> 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<ref name="Borenas" />  

 

== Inhibition, Regulation, and Future Pathways ==

The regulation of ALK dimerization by ALKAL points to one clear way of inhibiting ALK activity and may offer new therapeutic strategies in multiple disease settings <ref name="Li">PMID:34819665</ref>. As the dimerization of ALK is essential for activation of this protein, the inhibition of this activation is a potent way of inhibiting further ALK activity.<ref name ="Li" /> The inhibition and regulation of this extracellular region of ALK is actively being explored, as this part of ALK is part of what distinguishes it from other RTKs, like LTK. Researchers are currently exploring the use of [https://proteopedia.org/wiki/index.php/Antibody antibodies] and more specifically [https://proteopedia.org/wiki/index.php/Monoclonal_Antibody#:~:text=Monoclonal%20antibodies%20are%20immunoglobulins%20produced,pure%20homogeneous%20type%20of%20antibody. monoclonal antibodies]<ref name="Carpenter">PMID: 22266870</ref> as a means of inhibiting the activity of ALK through the extracellular domain. It is hypothesized that these monoclonal antibodies act by binding to the binding site of ALK, thus preventing ALKAL from binding<ref name="Li">, and thus induces cytotoxicity to the cancerous cell itself.<ref name ="Carpenter">

ALK can also be regulated in the extracellular domains by its own ligands. The different qualities of ALKAL1 and ALKAL2 make them both unique ligands for ALK, and both have different qualities in their interactions with ALK.