Background: KSR (kinase suppressor of Ras) is a recently identified putative protein kinase that positively mediates the Ras signaling pathway in the invertebrates Caenorhabditis elegans and Drosophila melanogaster. The function of vertebrate KSR is not well characterized biochemically or biologically.

Results: We examined the physiological role of KSR in vertebrate signal transduction using Xenopus laevis oocytes. Overexpression of KSR, in combination with overexpression of the intracellular dimeric protein 14-3-3, induced Xenopus oocyte meiotic maturation and cdc2 kinase activation; the effect of KSR and 14-3-3 on oocyte maturation was blocked by co-expression of dominant-negative Raf-1. We noted that KSR contains multiple potential binding sites for 14-3-3, and we used the yeast two-hybrid system and co-immunoprecipitation experiments to show that KSR can bind to 14-3-3. Furthermore, we demonstrated that KSR can form a complex with Raf kinase both in vitro and in cultured cells. Cell fractionation studies revealed that KSR formed a complex with 14-3-3 in both the membrane and cytoplasmic fractions of cell lysates; however, KSR only formed a complex with Raf-1 in the membrane fraction.

Conclusions: Our finding suggest that KSR, 14-3-3 and Raf form an oligomeric signaling complex and that KSR positively regulates the Ras signaling pathway in vertebrate organisms.