MPK12 in stomatal CO2 signaling: function beyond its kinase activity

O_LIProtein phosphorylation by kinases is a major molecular switch mechanism involved in the regulation of stomatal opening and closure. Previous research defined interaction between MAP kinase 12 and Raf-like kinase HT1 as a required step for stomatal movements by changes in CO2 concentration. However, whether MPK12 kinase activity is required for regulation of CO2-induced stomatal responses warrants in depth investigation. C_LIO_LIWe apply genetic, biochemical, and structural modeling approaches to examining the non-catalytic role of MPK12 in guard cell CO2 signaling that relies on allosteric inhibition of HT1. C_LIO_LIWe show that CO2/HCO3--enhanced MPK12 interaction with HT1 is independent of its phosphor-transfer activity. By analyzing gas exchange of plant lines expressing various kinase-dead and constitutively active versions of MPK12 in a plant line where MPK12 is deleted, we confirmed that CO2-dependent stomatal responses rely on MPK12s ability to bind to HT1 but not its kinase activity. We also demonstrate that purified MPK12 and HT1 proteins form a heterodimer in the presence of CO2/HCO3- and present structural modeling that explains the MPK12:HT1 interaction interface. C_LIO_LIThese data add to the model that MPK12 kinase-activity-independent interaction with HT1 functions as a molecular switch by which stomatal guard cells sense changes in atmospheric CO2 concentration. C_LI