Isolation of the protein kinase TAO2 and identification of its mitogen-activated protein kinase/extracellular signal-regulated kinase kinase binding domain

We previously reported the cloning of the thousand and one amino acid prote in kinase 1 (TAO1), a rat homolog of the Saccharomyces cerevisiae protein k inase sterile 20 protein. Here we report the complete sequence and properti es of a related rat protein kinase TAO2. Like TAO1, recombinant TAO2 select ively activated mitogen-activated protein/extracellular signal-regulated ki nase kinases (MEKs) 3, 4, and 6 of the stress-responsive mitogen-activated protein kinase pathways in vitro and copurified with MEK3 endogenous to Sf9 cells. To examine TAO2 interactions with MEKs, the MEK binding domain of T AO2 was localized to an similar to 135-residue sequence just C-terminal to the TAO2 catalytic domain. In vitro this MEK binding domain associated with MEKs 3 and 6 but not MEKs 1, 2, or 4, Using chimeric MEK proteins, we foun d that the MEK N terminus was sufficient for binding to TAO2. Catalytic act ivity of full-length TAO2 enhanced its binding to MEKs. However, neither th e autophosphorylation of the MEK binding domain of TAO2 nor the activity of MEK itself was required for MEK binding. These results suggest that TAO pr oteins lie in stress-sensitive kinase cascades and define a mechanism by wh ich these kinases may organize downstream targets.