DictyosteliumStress-activated Protein Kinase α, a Novel Stress-activated Mitogen-activated Protein Kinase Kinase Kinase-like Kinase, Is Important for the Proper Regulation of the Cytoskeleton

Abstract

Mitogen-activated protein kinase cascades regulate various cellular functions, including growth, cell differentiation, development, and stress responses. We have identified a new Dictyostelium kinase (stress-activated protein kinase [SAPK]α), which is related to members of the mixed lineage kinase class of mitogen-activated protein kinase kinases. SAPKα is activated by osmotic stress, heat shock, and detachment from the substratum and by a membrane-permeable cGMP analog, a known regulator of stress responses in Dictyostelium. SAPKα is important for cellular resistance to stresses, because SAPKα null cells exhibit reduced viability in response to osmotic stress. We found that SAPKα mutants affect cellular processes requiring proper regulation of the actin cytoskeleton, including cell motility, morphogenesis, cytokinesis, and cell adhesion. Overexpression of SAPKα results in highly elevated basal and chemoattractant-stimulated F-actin levels and strong aggregation and developmental defects, including a failure to polarize and chemotax, and abnormal morphogenesis. These phenotypes require a kinase-active SAPKα. SAPKα null cells exhibit reduced chemoattractant-stimulated F-actin levels, cytokinesis, developmental and adhesion defects, and a motility defect that is less severe than that exhibited by SAPKα-overexpressing cells. SAPKα colocalizes with F-actin in F-actin–enriched structures, including membrane ruffles and pseudopodia during chemotaxis. Although SAPKα is required for these F-actin–mediated processes, it is not detectably activated in response to chemoattractant stimulation.