A Mek1-Mek2 heterodimer determines the strength and duration of the Erk signal.

Mek1 and Mek2 are kinases that phosphorylate Erk, participating in the signal transduction pathway controlling cellular growth and adhesion. Though closely related, there are clear functional differences, with Mek1 being subject to negative-feedback regulation via phosphorylation by Erk. Now Mek1 and Mek2 are shown to form a heterodimer in vivo, in which Mek2 activity is also controlled by Erk phosphorylation of Mek1.