Trib1 interaction domains in MEK1. (A) Constructs of MEK1 mutants. BD indicates binding domain. (B) U2OS cells were transiently transfected with FLAG-tagged Trib1 and HA-tagged MEK1. The cell lysates were immunoprecipitated with an anti-FLAG polyclonal antibody and immunoblotted with an anti-HA monoclonal antibody. The expression level of each protein was assessed by immunoblotting whole cell lysates with anti-HA or anti-FLAG antibodies. ACT indicates activation domain; and PRD, proline-rich domain