FRET-based detection of different conformations of MK2.

MAP kinase-activated protein kinase 2 (MK2 or MAPKAP K2) is a stress-activated enzyme downstream to p38 MAPK. By fusion of green fluorescent protein variants to the N- and C-terminus we analysed conformational changes in the kinase molecule in vitro and in vivo. Activation of MK2 is accompanied by a decrease in fluorescence resonance energy transfer, indicating a transition from an inactive/closed to an active/open conformation with an increase in the apparent distance between the fluorophores of approximately 9 A. The closed conformation exists exclusively in the nucleus. Upon stress, the open conformation of MK2 rapidly becomes detectable in the cytoplasm and accumulates in the nucleus only when Crm1-dependent nuclear export is blocked. Hence, in living cells activation of MK2 and its nuclear export are coupled by a phosphorylation-dependent conformational switch.