(A) Level of IgM surface expression. ITAM of K1 was required for its signal-transducing activity. Furthermore, tyrosine residues of the putative ITAM of K1 were phosphorylated upon activation, and this allowed subsequent binding of SH2-comprising proteins. These results demonstrate the KSHV transforming protein K1 contains a functional ITAM in its cytoplasmic website and that it can transduce signals to induce cellular activation. Engagement of the B-cell antigen receptor (BCR) and the T-cell antigen receptor (TCR) initiates multiple intracellular signals that can lead to cellular proliferation and the acquisition of complex effector functions. Analysis of sequence elements responsible for the signaling properties of the transducing subunits Temsirolimus (Torisel) of BCR and TCR offers led to the identification of the immunoreceptor tyrosine-based activation motif (ITAM) (3, 8, 36, 37). This motif consists of six conserved amino acid residues spaced exactly over an 26-amino acid sequence, (D/E)X7(D/E)X2YX2LX7YX2L/I, where X is definitely any amino acid. The ITAM is present in a number of cellular signal-transducing molecules, such as TCR-, immunoglobulin alpha (Ig), Ig, CD3, CD3, Fc?RI, bovine leukemia disease gp30, Epstein-Barr disease (EBV) LMP2A, and simian immunodeficiency disease PBj14 Nef (2, 3, 7, 8, 22, 23). It has been well recorded that this motif is necessary and adequate for the coupling of extracellular signals to intracellular signaling molecules. Upon activation, the tyrosine residues within the ITAMs become phosphorylated, permitting the binding of SH2 domain-containing proteins. Subsequent signaling molecules are recruited to these connected proteins via SH2 or additional modular connection domains (3, 8, 33, 37). The association of ITAM-containing receptors with tyrosine kinases has been analyzed in substantial detail (32). Immediately downstream of the BCR and the TCR in the signaling pathway are the receptor-associated protein tyrosine kinases (PTKs) (3, 36, 37). Two families of PTKs have been shown to be involved in BCR and TCR signaling. Lyn, Fyn, Blk, and Lck are users of Src family, while Zap70 and Syk make up another PTK family (3, 36, 37). A primary role of the Src family kinases is usually to phosphorylate two tyrosine residues within the ITAMs of BCR and TCR. Subsequently, Syk and Zap70 are recruited to activated receptors, which in turn lead to the induction of cellular tyrosine phosphorylation, the elevation of intracellular calcium, the activation of lipid-dependent kinases, and the activation of Ras and its downstream kinase cascade (3, 36, 37). The cross-linking of chimeric molecules composed of the extracellular and transmembrane Bglap domains of the CD4, CD8, or CD16 molecule and a single copy of the ITAM motif has been shown to be sufficient to elicit early or late signal-transducing events (2, 12, 15, 17). Thus, ITAMs function as a scaffold to recruit and organize effector molecules upon receptor ligation. DNA sequences of a novel member of the herpesvirus group, called Kaposis sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8, have been widely recognized in Karposis sarcoma tumors from human immunodeficiency virus-positive and -unfavorable patients (4, 5, 21). KSHV has also been recognized in body cavity-based lymphoma and some forms of Castlemans disease (4, 5, 29). The genomic sequence shows indicates KSHV to be a gammaherpesvirus that is closely related to herpesvirus saimiri (HVS) (26, 30) and the recently isolated rhesus monkey rhadinovirus (6). DNA sequence analysis of the entire 140.5 kbp of the KSHV genome revealed a number of cellular homologs which could possibly contribute to the pathogenesis associated with this virus (26, 30). These include a virus-encoded interleukin-6 (IL-6) (24, 27, 28), MIP1-/ chemokines (14, 24, 28), a Bcl-2 homolog (31), Temsirolimus (Torisel) a virus-encoded interferon regulatory factor (9, 19, 38), v-cyclin (10, 20), IL-8 receptor (1), FLICE-inhibitory protein (35), and an N-CAM homolog. At a position equivalent to the STP (saimiri transformation protein) of HVS, KSHV contains a distinct open reading frame called K1 (18). Although KSHV and HVS are related users of the rhadinovirus subgroup of gammaherpesviruses, K1 and STP exhibit no similarity in amino acid sequence or in business of structural motif. The K1 protein Temsirolimus (Torisel) is predicted to have a transmission peptide sequence at the amino terminus, an extracellular domain name, a transmembrane domain name, and a short cytoplasmic tail at the carboxyl terminus (16, 18, 30). The predicted extracellular domain name of the K1 protein apparently influences the strength or degree of disulfide-linked oligomerization (18). Expression of the K1 gene in rodent fibroblasts produced morphologic changes and focus formation indicative of transformation. A recombinant herpesvirus in which the STP oncogene of HVS was replaced with the K1 gene immortalized main T.