FCERI aggregation has been shown to activate JNK as well as protein kinases upstream of JNK, such as MEKK1 (Mitogen-activated protein kinase/ERK Kinase Kinase-1) and JNK kinase (JNKK). PAK has been shown to be the upstream kinase involved in the activation of MEKK1, however no direct phosphorylation of MEKK1 by PAK is observed. Two threonine residues at positions 1400 and 1412 (analogous to 1381 and 1393 in mouse) in the activation loop of MEKK1 between the kinase subdomains VII and VIII are essential for its catalytic activity. The catalytic domain of MEKK1 is able to autophosphorylate these residues, enhancing its own activity.