The aberrant kinase activity of RET ( re arranged during t ransfection), a transmembrane tyrosine kinase, is associated with human cancer. A point mutation caused by the replacement of solvent-front hydrophilic S904, located on the activation loop (A-loop), with a bulky hydrophobic phenylalanine residue can induce resistance to the type I kinase inhibitor vandetanib. A possible mechanism of this drug resistance is the release of a cis-autoinhibited conformation of RET for autophosphorylation, which activates RET kinase. Because the association between S904F mutation and enhanced autophosphorylation is unclear, we conducted molecular modeling analysis to compare unphosphorylated apo wild-type and S904F mutant structures. The structural compactness of the A-loop promoted ATP binding. When the A-loop is extended, the αC helix moves toward the glycine-rich loop, resulting in the protrusion of F735. The extruded F735 connects with E734 and R912 and constrains the ATP pocket entrance. Contrarily, a contracted A-loop pulls the αC helix away from the glycine-rich loop, burying F734 and making the ATP pocket accessible. The mutated F904 stabilizes the contracted A-loop and releases the autoinhibited conformation of RET, thereby facilitating autophosphorylation. We also simulated two ATP-bound systems. The binding free energies of ATP, estimated through the molecular mechanics with a generalized Born and surface area solvation approach, revealed that the S904F mutant was bound more tightly than was the wild type with the ATP. The findings support the premise of autophosphorylation promotion in the S904F mutant.
