A Conserved Isoleucine Maintains the Inactive State of Bruton's Tyrosine Kinase

• Tec family kinases mediate the immune response and contain catalytic domains that are inactive by default.
• Molecular dynamics simulations identify an isoleucine in the N-lobe that maintains the inactive state of the Btk kinase.
• Conservative mutation of isoleucine to leucine abolishes the “inactive by default” status of the Btk kinase domain.
• Unique conformational requirements of the β-branched isoleucine side chain create a wedge that sterically prevents sampling of the active state; mutation to a leucine side chain removes the steric impediment, allowing increased population of the active conformation.

Despite high level of homology among non-receptor tyrosine kinases, different kinase families employ a diverse array of regulatory mechanisms. For example, the catalytic kinase domains of the Tec family kinases are inactive without assembly of the adjacent regulatory domains, whereas the Src kinase domains are autoinhibited by the assembly of similar adjacent regulatory domains. Using molecular dynamics simulations, biochemical assays, and biophysical approaches, we have uncovered an isoleucine residue in the kinase domain of the Tec family member Btk that, when mutated to the closely related leucine, leads to a shift in the conformational equilibrium of the kinase domain toward the active state. The single amino acid mutation results in measureable catalytic activity for the Btk kinase domain in the absence of the regulatory domains. We suggest that this isoleucine side chain in the Tec family kinases acts as a “wedge” that restricts the conformational space available to key regions in the kinase domain, preventing activation until the kinase domain associates with its regulatory subunits and overcomes the energetic barrier to activation imposed by the isoleucine side chain.

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