The threonine residues in MAP kinase kinase 1 phosphorylated by MAP kinase in vitro are also phosphorylated in nerve growth factor‐stimulated rat phaeochromocytoma (PC12) cells

Abstract
The residues on MAP kinase kinase‐1 (MAPKK1) phosphorylated by MAP kinase in vitro have been identified as Thr‐291 and Thr‐385. Both threonines are phosphorylated in PC12 cells and the 32P‐labelling of each residue increases after stimulation with nerve growth factor (NGF). The results establish that MAPKK1 is a physiological substrate for MAP kinase. The two active forms of MAPKK that are resolved by Mono Q chromatography of PC12 cell extracts are both phosphorylated at Thr‐291 and Thr‐385, demonstrating that neither species is the MAPKK2 isoform which lacks Thr‐291.