Crosslinking of membrane-bound IMmuno- (5-7). Associations of the interleukin 2 receptor f3 chain and T-cell antigen receptor with the Src-like kinases Lck and Fyn, respectively, were also reported (8-10). However, the physiological substrates or targets of these Src-like kinases in receptor-mediated signaling have yet to be identified. In recent studies on the targets of the tyrosine kinases, much information has been obtained about the plateletderived growth factor receptor (PDGF-R). The binding of PDGF to PDGF-R (J3 type) activates the receptor's kinase activity and induces its association with tyrosine-phosphorylated proteins such as phosphatidylinositol 3-kinase (PI 3-kinase), Ras GTPase-activating protein, and phospholipase C-y upon ligand-mediated signaling (11). One possible target of the tyrosine kinase, PI 3-kinase, is also associated with ligand-stimulated receptors for colony-stimulating factor 1 and insulin (11)(12)(13) and with an activated form of Src, p60-src (14). The levels ofproducts of PI 3-kinase are elevated in cells carrying these activated tyrosine kinases. Therefore, PI 3-kinase is likely to be an important target oftyrosine kinases.The lyn gene is a member of the src family and encodes two forms of protein-tyrosine kinase (Lyn), p531Yfl and p56IYn. 1118The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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Three monoclonal antibodies were prepared against rat brain soluble protein kinase C. Two of the antibodies, CKI-97 (IgG2b subclass) and CKII-90 (IgG1 subclass), showed weak binding to native protein kinase C. The third antibody, CKI-33 (IgG2b subclass), showed no binding. However, the mixture of CKI-97, CKII-90, and CKI-33 exhibited much stronger binding activity to this protein kinase than any of the antibodies alone. Although none of these antibodies showed protein kinase C-neutralizing activity, Western blot analysis indicated that these antibodies reacted specifically with protein kinase C, presumably its subspecies, that is present predominantly in nervous tissues. Immunocytochemical studies shows that these antibodies can be used for identification of this enzyme in nervous tissues. In rat Purkinje cells, the immunoreactive material was present throughout the cytoplasm, including dendrites and axons, but was poorly represented in the cell nucleus. In cerebellum, the localization of protein kinase C appears to be very similar to that of cGMP-dependent protein kinase.