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Background
Anaplastic Lymphoma Kinase (ALK) is a member of the family of Receptor Tyrosine Kinases (RTKs), a family of biomolecules that are primarily responsible for biosignaling pathways such as the insulin signaling pathway. It was discovered 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
As previously mentioned, ALK is a close homolog of LTK, and together these two homologous proteins create a subgroup within the superfamily of insulin receptors (IR). ALK is composed of 1620 amino acids in total, with three primary domains. ALK is described as showing the "classic structural features of RTKs, with an extracellular ligand-binding domain, a transmembrane-spanning region, and an intracellular kinase domain." As displayed in (insert line figure here), the intracellular tyrosine kinase domain ranges from residues 1116-1392, and features the C-terminal end. 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 glycine-rich domain (GlyR), a tumor necrosis factor-like (TNF-like), and an epidermal growth factor-like domain (EGF-like). All four domains of this subregion of the extracellular region contribute to ligand-binding, (more info here).
Function
Poly-Glycine Domain
- glycine helices
Disease
ALKAL 1
ALKAL 1 (Anaplastic Lymphoma Kinase Ligand 1) is a monomeric ligand of ALK, in addition to ALKAL 2. Structurally, ALKAL 1 and ALKAL 2 contain an N-terminal variable region and a conversed C-terminal augmentor domain. However, in ALKAL 1, this N-terminal variable region is shorter, and shares no similar sequences to ALKAL 2. Nevertheless, ALKAL 1 shares a 91% sequence similarity with ALKAL 2. Both ligands include a three helix bundle domain in their structures, with an extended positively charged surface which is used in ligand binding.
Relevance
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