Phosphoregulation of mixed-lineage kinase 1 activity by multiple phosphorylation in the activation loop

Mixed-lineage kinase 1 (MLK1) is a mitogen-activated protein kinase kinase kinase capable of activating the c-Jun NH2-terminal kinase (JNK) pathway. Immunoblot and mass spectrometry show that MLK1 is threonine phosphorylated in or near the activation loop. A kinase dead mutant is not, consistent with autophosphorylation.