Structure of Escherichia coli tyrosine kinase Etk reveals a novel activation mechanism

While protein tyrosine (Tyr) kinases (PTKs) have been extensively characterized in eukaryotes, far less is known about their emerging counterparts in prokaryotes. The inner‐membrane Wzc/Etk protein belongs to the bacterial PTK family, which has an important function in regulating the polymerization and transport of virulence‐determining capsular polysaccharide (CPS). The kinase uses a unique two‐step activation process involving intra‐phosphorylation of a Tyr residue, although the molecular mechanism remains unknown. Herein, we report the first crystal structure of a bacterial PTK, the C‐terminal kinase domain of Escherichia coli Tyr kinase (Etk) at 2.5‐Å resolution. The fold of the Etk kinase domain differs markedly from that of eukaryotic PTKs. Based on the observed structure and supporting mass spectrometric evidence of Etk, a unique activation mechanism is proposed that involves the phosphorylated Tyr residue, Y574, at the active site and its specific interaction with a previously unidentified key Arg residue, R614, to unblock the active site. Both in vitro kinase activity and in vivo antibiotics resistance studies using structure‐guided mutants further support the novel activation mechanism.