Mechanism of activation of an ancestral Tec kinase by PIP3

The TEC kinases are a family of five paralogous mammalian genes that play crucial roles in cell growth, proliferation and differentiation, particularly in immune cells. The recruitment and activation of the TEC kinases depend on the generation of the lipid second messenger, PIP3, in the plasma membrane. However, the mechanisms by which PIP3 activates the TEC kinases are not well understood. We have elucidated the autoinhibited conformation of an ancestral TEC kinase from the choanoflagellate Monosiga brevicollis. We demonstrate that PIP3 relieves autoinhibition of MbTEC by displacing its PH domain from an evolutionarily conserved inhibitory interaction with its kinase domain. We also show that a conserved polyproline motif within MbTEC promotes its activation in a kinase-intrinsic mechanism. Finally, we show that the PH domain is sufficient to restore autoinhibition in a constitutively active mutant of MbTEC. Our findings reveal that PIP3 is necessary and sufficient for both MbTEC activation and inactivation. Significance StatementThe Tec family of protein kinases plays an essential role in cell signaling, particularly in the proliferation and differentiation of immune cells. Consequently, their dysregulation is causative of inherited immunodeficiency, while the Tec kinases are also therapeutic targets in the control of hematological malignancies. We have elucidated a conserved mechanism by which the Tec kinases are activated by the lipid second messenger PIP3. PIP3 is necessary and sufficient for Tec activation, while its turnover is sufficient for Tec inactivation. Our work identifies PIP3 as the ultimate gatekeeper of Tec activity in cells, with implications for the rationalization and treatment of human disease.