Recombinant human MAP3K5 (ASK1) is needed for phosphorylation of recombinant mouse Map2k4 (Sek1) when both proteins are exogenously expressed in COS7 cells. The direct phosphorylation of Map2k4 by MAP3K5 was demonstrated in vitro, using recombinant human MAP3K5 and recombinant Xenopus Map2k4 (Ichijo et al. 1997).
ten Dijke, P,
protein serine/threonine kinase activity of MAP3K5 [cytosol]