Differential activation of p38 mitogen-activated protein kinase isoforms depending on signal strength

We have investigated the ability of the mitogen-activated protein kinase (MAPK) kinase MKK6 to activate different members of the p38 subfamily of MAPKs and found that some MKK6 mutants can efficiently activate p38 alpha but not p38 gamma. In contrast, a constitutively active MKK6 mutant activated both p38 MAPK isoforms to similar extents. The same results were obtained upon coexpression in Xenopus oocytes and in vitro using either MKK6 immunoprecipitates from transfected cells or bacterially produced recombinant proteins. We also found that the preferential activation of p38 alpha by MKK6 correlated with more efficient binding of MKK6 to p38 alpha than to p38 gamma. Furthermore, increasing concentrations of constitutively active MKK6 differentially activated either p38 alpha alone (low MKK6 activity) or both p38 alpha and p38 gamma (high MKK6 activity), both in vitro and in injected oocytes. The determinants for selectivity are located at the carboxyl-terminal lobe of p38 MAPKs but do not correspond to the activation loop or common docking sequences. We also showed that different stimuli can induce different levels of endogenous MKK6 activity that correlate with differential activation of p38 MAPKs. Our results suggest that the level of MKK6 activity triggered by a given stimulus may determine the pattern of downstream p38 MAPK activation in the particular response.