Expression of kinase suppressor of Ras in the normal adult and embryonic mouse.

Recent studies indicate that kinase suppressor of Ras (KSR)is a scaffold protein for the Ras/Raf/MEK/ERK signaling cascade in mammals. To help determine the in vivo function of KSR, we have examined the tissue-specific distribution of this protein in the embryonic and adult mouse using a rat monoclonal antibody raised against the mouse protein. Western blot analysis indicates that the protein is expressed at highest levels in the adult brain. It is also expressed at low levels in bladder, ovary, testis, and lung, but the protein is not detectable in any other adult tissue. However, reverse transcription-PCR analysis shows that Ksr transcripts are detected in all adult tissues except the liver. A variant containing a differentially spliced exon in the CA4 domain is observed in brain, cerebellum, ovary, and intestine. The protein is also expressed throughout the E6.5 embryo and at high levels in the neuroepithelium of the E10.5 embryo. At this embryonic stage, expression is also detected at lower levels in the limb and tail buds as well as in the myocardium.