Protein kinase; unclassified specificity. Phosphotransferases. The specificity of this class of kinases can not be predicted. Possible dual-specificity Ser/Thr/Tyr kinase.

Tyrosine kinase, catalytic domain. Phosphotransferases. Tyrosine-specific kinase subfamily.

Protein tyrosine kinase.

Catalytic domain of Protein Tyrosine Kinases. Protein Tyrosine Kinase (PTK) family, catalytic domain. This PTKc family is part of a larger superfamily that includes the catalytic domains of protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase (PI3K). PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. They can be classified into receptor and non-receptor tyr kinases. PTKs play important roles in many cellular processes including, lymphocyte activation, epithelium growth and maintenance, metabolism control, organogenesis regulation, survival, proliferation, differentiation, migration, adhesion, motility, and morphogenesis. Receptor tyr kinases (RTKs) are integral membrane proteins which contain an extracellular ligand-binding region, a transmembrane segment, and an intracellular tyr kinase domain. RTKs are usually activated through ligand binding, which causes dimerization and autophosphorylation of the intracellular tyr kinase catalytic domain, leading to intracellular signaling. Some RTKs are orphan receptors with no known ligands. Non-receptor (or cytoplasmic) tyr kinases are distributed in different intracellular compartments and are usually multi-domain proteins containing a catalytic tyr kinase domain as well as various regulatory domains such as SH3 and SH2. PTKs are usually autoinhibited and require a mechanism for activation. In many PTKs, the phosphorylation of tyr residues in the activation loop is essential for optimal activity. Aberrant expression of PTKs is associated with many development abnormalities and cancers.

Di-glucose binding within endoplasmic reticulum. Malectin is a membrane-anchored protein of the endoplasmic reticulum that recognises and binds Glc2-N-glycan. It carries a signal peptide from residues 1-26, a C-terminal transmembrane helix from residues 255-274, and a highly conserved central part of approximately 190 residues followed by an acidic, glutamate-rich region. Carbohydrate-binding is mediated by the four aromatic residues, Y67, Y89, Y116, and F117 and the aspartate at D186. NMR-based ligand-screening studies has shown binding of the protein to maltose and related oligosaccharides, on the basis of which the protein has been designated "malectin", and its endogenous ligand is found to be Glc2-high-mannose N-glycan.