A wortmannin-sensitive and insulin-stimulated protein kinase (WISK), which phosphorylates and activates cardiac 6-phosphofructo-2-kinase (PFK-2), was partially purified from perfused rat hearts. Immunoblotting showed that WISK was devoid of protein kinase B (PKB), serum- and glucocorticoid-regulated protein kinase and protein kinase Czeta (PKCzeta). Comparison of the inhibition of WISK, PKCalpha and PKCzeta by different protein kinase inhibitors suggested that WISK was not a member of the PKC family. In addition, WISK contained no detectable phosphoinositide-dependent protein kinase-1 (PDK1) activity. WISK phosphorylated recombinant heart PFK-2 in a time-dependent manner to the extent of 0.4 mol of phosphate incorporated/mol of enzyme subunit, and increased the V(max) of PFK-2 twofold, without affecting the K(m) for fructose 6-phosphate. WISK phosphorylated Ser-466 to a greater extent than Ser-483 in recombinant heart PFK-2, and both sites were demonstrated to be phosphorylated to the same extent by PKB. Gel filtration and in-gel kinase analysis indicated that WISK was a monomer with a M(r) of 56500. Treatment of WISK with protein phosphatase 2A (PP2A) catalytic subunits reversed the effect of insulin, suggesting the involvement of an upstream activating kinase. Indeed, PDK1 was able to partially reactivate the PP2A-treated WISK and this reactivation was not enhanced by PtdIns(3,4,5)P(3)-containing vesicles. Moreover, a single 57000-M(r) band was labelled on incubation of the dephosphorylated WISK preparation with PDK1 and [gamma-(32)P]ATP. These findings provide evidence for the existence of a new protein kinase in the insulin signalling pathway, probably downstream of PDK1.