THE DICTYOSTELIUM MITOGEN-ACTIVATED PROTEIN-KINASE ERK2 IS REGULATED BY RAS AND CAMP-DEPENDENT PROTEIN-KINASE (PKA) AND MEDIATES PKA FUNCTION

The chemoattractant cAMP, acting through serpentine cAMP receptors, re sults in a rapid and transient stimulation of the Dictyostelium mitoge n-activated protein kinase ERK2 activity (1), In this study we show th at other pathways required for aggregation, including Ras and cAMP-dep endent protein kinase (PKA), are important regulators of ERK2 activati on and adaptation. By examining both the level and kinetics of activat ion and adaptation of ERK2, we show that Ras is a negative regulator o f ERK2. Activated Ras or disruption of a Ras GAP gene results in reduc ed ERK2 activation whereas disruption of putative Ras GEF or expressio n of dominant negative Res proteins have a more rapid, higher, and ext ended activation. CRAC, a PH domain-containing protein required for ad enylyl cyclase activation, is also required for proper ERK2 adaptation . PKA overexpression results in a more rapid, higher level of activati on, whereas pka null cells show a lower level but more extended ERK2 a ctivation. Furthermore, we show that constitutive expression of PKA ca talytic subunit bypasses the requirement of ERK2 for aggregation and l ater development, indicating that PKA lies downstream from ERK2 and th at ERK2 may regulate one or more components of the signaling pathway r equired for mediating PRA function, possibly by directly regulating PK A R or a protein controlling the intracellular level of cAMP.