IntroductionKit, the receptor for stem cell factor (SCF), is a type III receptor tyrosine kinase of the platelet-derived growth factor (PDGF) subfamily. c-kit maps to the white spotting (W) locus in the mouse, 1,2 while the gene for its ligand SCF maps to the steel (Sl) locus. 3,4 Mutations that reduce function or expression of either the receptor or ligand lead to macrocytic anemia, mast cell deficiency, abnormal pigmentation of skin and hair, infertility, reduced gastrointestinal motility, and impaired hippocampal learning. 5 Gain-offunction mutations in Kit can be oncogenic and are found in various diseases including leukemias, mastocytosis, germ cell tumors, and gastrointestinal stromal tumors (GISTs). 6 One common gain-of-function mutation is the substitution of valine, tyrosine, asparagine, or histidine for aspartic acid 816 in the catalytic domain of human Kit. Mutations of codon 816 have been identified in patients with mastocytosis, 7-10 acute myeloid leukemia (AML), [11][12][13] and germ cell tumors. 14 Mutations of the corresponding codon (D814 in mice, D817 in rats) have been found in a murine mastocytoma cell line P-815 15 and a rat mast cell tumor line RBL-2H3. 16 Expression of D814Y/V Kit in the murine mast cell-like line IC2, as well as several other factor-dependent hematopoietic cell lines, results in factor-independent growth and tumorigenesis. [17][18][19][20] Interestingly, cells expressing a Kit catalytic domain mutant are resistant to STI571/imatinib (Gleevec), an inhibitor of wild-type (WT) Kit. [21][22][23] Thus, drugs targeting this mutant receptor or its downstream effectors could be useful therapeutic agents in the treatment of diseases associated with this mutation.The protein kinase C (PKC) family of serine-threonine protein kinases encompasses at least 12 members, 24 which can be separated into 3 classes: conventional, novel, and atypical. The conventional PKCs include the ␣, , and ␥ isoforms. Activation of these PKCs requires both calcium ions and a lipid, such as diacylglycerol (DAG) or phosphatidylserine, or a phorbol ester. The novel PKCs, including ␦, ⑀, , and isoforms, are calcium-independent and require only a lipid or a phorbol ester for kinase activity. Unlike the conventional isoforms, the novel isoforms can be activated by the products of phosphatidylinositol 3-kinase (PI3K), PtdIns-3,4-P 2 , or PtdIns-3,4,5-P 3, as well as DAG or phosphatidylserine. 25 The atypical PKCs, including the and isoforms, require neither calcium nor a lipid or phorbol ester for kinase activity.PKC isoforms are reported to phosphorylate wild-type (WT) Kit on serine residues after stimulation with SCF. Previous work has shown that this reduces autophosphorylation activity and subsequent interaction with a variety of signaling components. 26,27 Importantly, these studies were performed in porcine aortic endothelial cells, which predominantly express the conventional PKC isoforms ␣ and . The role of novel PKC isoforms in signaling through WT Kit has not been addressed. Further, little is known a...