THE MIXED LINEAGE KINASE SPRK PHOSPHORYLATES AND ACTIVATES THE STRESS-ACTIVATED PROTEIN-KINASE ACTIVATOR, SEK-1

SPRK (also called PTK-1 and MLK-3), a member of the mixed lineage kina se subfamily of (Ser/Thr) protein kinases, encodes an amino-terminal S H3 domain followed by a kinase catalytic domain, two leucine zippers i nterrupted by a short spacer, a Rac/Cdc42 binding domain, and a long c arboxyl-terminal proline-rich region. We report herein that SPRK activ ates the stress-activated protein kinases (SAPKs) but not ERK-1 during transient expression in COS cells; the p38 kinase is activated modest ly (1.3-2 fold) but consistently. SPRK also activates cotransfected SE K 1/MKK-4, a dual specificity kinase which phosphorylates and activate s SAPK. Reciprocally, expression of mutant, inactive SEK-1 inhibits co mpletely the basal and SPRK-activated SAPK activity. Immunoprecipitate d recombinant SPRK is able to phosphorylate and activate recombinant S EK-1 in vitro to an extent comparable to that achieved by MEK kinase-1 . These results identify SPRK as a candidate upstream activator of the stress-activated protein kinases, acting through the phosphorylation and activation of SEK-1.