(a) The BTK fragments used in this study are shown with key residues indicated above each domain. PHTH, Pleckstrin homology-Tec homology domain; PRR, proline-rich region; SH3, Src homology three domain; SH2, Src homology two domain; L, SH2-kinase linker; FL, full-length; 32LKD, SH3-SH2-linker-kinase domain; and LKD, linker-kinase domain. (b) Domain arrangement of BTK full-length (FL) in the autoinhibited conformation (left) based on the crystal structure of the BTK SH3-SH2-kinase (32LKD) fragment (PDB ID: 4XI2) and solution data supporting an autoinhibitory interaction between the PHTH domain and the activation loop face of the kinase domain (Amatya et al., 2019; Devkota et al., 2017). The location of T316 in the SH2 domain is indicated with a black circle. The active LKD fragment is depicted on the right. (c) The thiol group of C481 in the BTK kinase domain forms a covalent bond with the α,β unsaturated ketone of Ibrutinib. (d) Crystal structure (PDB ID: 5P9J) of BTK kinase domain (gray cartoon) bound to Ibrutinib, represented in pink sticks. The kinase domain N- and C-lobes are labeled, the activation loop is purple, the αC-helix yellow, side chains of the Lys/Glu pair are shown in cyan, sidechains of W395, Y551, and C481 are shown in yellow spheres, and the regulatory spine (R-spine) residues are shown as blue sticks and transparent surface.