Pseudokinase domain of the Protein Tyrosine Kinase, Colon Carcinoma Kinase 4
CCK4, also called protein tyrosine kinase 7 (PTK7), is an orphan receptor PTK (RTK) containing an extracellular region with seven immunoglobulin domains, a transmembrane segment, and an intracellular inactive pseudokinase domain, which shows similarity to tyr kinases but lacks crucial residues for catalytic activity and ATP binding. Studies in mice reveal that CCK4 is essential for neural development. Mouse embryos containing a truncated CCK4 die perinatally and display craniorachischisis, a severe form of neural tube defect. The mechanism of action of the CCK4 pseudokinase is still unknown. Other pseudokinases such as HER3 rely on the activity of partner RTKs. The CCK4 subfamily is part of a larger superfamily that includes other pseudokinases and the catalytic domains of active kinases including PTKs, protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.