MEKK1 PHOSPHORYLATES MEK1 AND MEK2 BUT DOES NOT CAUSE ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASE

A constitutively active fragment of rat MEK kinase 1 (MEKK1) consistin g of only its catalytic domain (MEKK-C) expressed in bacteria quantita tively activates recombinant mitogen-activated protein (MAP) kinase/ex tracellular signal-regulated protein kinase (ERK) kinases 1 and 2 (MEK 1 and MEK2) in vitro. Activation of MEK1 by MEKKI-C is accompanied by phosphorylation of S218 and S222, which are also phosphorylated by the protein kinases c-Mos and Raf-1. MEKK1 has been implicated in regulat ion of a parallel but distinct cascade that leads to phosphorylation o f N terminal sites on c-Jun; thus, its role in the MAP kinase pathway has been questioned. However, in addition to its capacity to phosphory late MEK1 in vitro, MEKK-C interacts with MEK1 in the two-hybrid syste m, and expression of mouse MEK1 or MEKK-C in mammalian cells causes co nstitutive activation of both MEK1 and MEK2. Neither cotransfected nor endogenous ERK2 is highly activated by MEKK1 compared to its stimulat ion by epidermal growth factor in spite of significant activation of e ndogenous MEK. Thus, other as Set undefined mechanisms may be involved in determining information flow through the MAP kinase and related pa thways.