THE DICTYOSTELIUM MAP KINASE ERK2 REGULATES MULTIPLE, INDEPENDENT DEVELOPMENTAL PATHWAYS

We showed previously that the MAP kinase ERK2 is essential for aggrega tion, erk2 null cells lack cAMP stimulation of adenylyl cyclase and th us cannot relay the cAMP chemotactic signal, although the cells chemot ax to cAMP (Segall et al. 1995). In this paper we have examined the ro le of ERK2 in controlling developmental gene expression and morphogens is during the multicellular stages, making use of a temperature-sensit ive ERK2 mutation. Using suspension assays, we show that ERK2 is not e ssential for aggregation-stage, cAMP pulse-induced gene expression, or for the expression of postaggregative genes, which are induced at the onset of mound formation in response to cAMP in wild-type cells. In c ontrast, the prespore-specific gene SP60 is not induced and the presta lk-specific gene ecmA is induced but at a significantly reduced level. Chimeric organisms, comprised of wild-type and erk2 null cells expres sing the prestalk-specific scmA/lacZ reporter, show an abnormal spatia l patterning, in which Erk2(ts)/erk2 cells are excluded from the very anterior prestalk A region. To further examine the function of ERK2 du ring the multicellular stages, we bypassed the requirement of ERK2 for aggregation by creating an ERK2 temperature-sensitive mutant. erk2 nu ll cells expressing the ERK2(ts) mutant develop normally at 20 degrees C and express cell-type-specific genes but do not aggregate at temper atures above 25 degrees C. Using temperature shift experiments, we sho wed that ERK2 is essential for proper morphogenesis and for the induct ion and maintenance of prespore but not prestalk gene expression. Our results indicate that ERK2 functions at independent stages during Dict yostelium development to control distinct developmental programs: duri ng aggregation, ERK2 is required for the activation of adenylyl cyclas e and during multicellular development, ERK2 is essential for morphoge nesis and cell-type-specific gene expression. Analysis of these result s and others supports the conclusion that the requirement of ERK2 for cell-type differentiation is independent of its role in the activation of adenylyl cyclase.