Identification of an Immunoreceptor Tyrosine-Based Activation Motif of K1 Transforming Protein of Kaposi’s Sarcoma-Associated Herpesvirus

Abstract

Kaposi's sarcoma-associated herpesvirus (KSHV) is consistently identified in Kaposi's sarcoma and body cavity-based lymphoma. KSHV encodes a transforming protein called K1 which is structurally similar to lymphocyte receptors. We have found that a highly conserved region of the cytoplasmic domain of K1 resembles the sequence of immunoreceptor tyrosine-based activation motifs (ITAMs). To demonstrate the signal-transducing activity of K1, we constructed a chimeric protein in which the cytoplasmic tail of the human CD8␣ polypeptide was replaced with that of KSHV K1. Expression of the CD8-K1 chimera in B cells induced cellular tyrosine phosphorylation and intracellular calcium mobilization upon stimulation with an anti-CD8 antibody. Mutational analyses showed that the putative ITAM of K1 was required for its signal-transducing activity. Furthermore, tyrosine residues of the putative ITAM of K1 were phosphorylated upon stimulation, and this allowed subsequent binding of SH2-containing proteins. These results demonstrate that the KSHV transforming protein K1 contains a functional ITAM in its cytoplasmic domain and that it can transduce signals to induce cellular activation.

Cellular ITAM-containing proteins are oncoproteins in nonhematopoietic cells

“…However, since their identification, it has been noted that a number of oncogenic viruses encode transmembrane ITAMcontaining proteins. These proteins include EpsteinBarr virus (EBV) LMP2A, murine mammary tumor virus (MMTV) Env, and Kaposi's sarcoma-associated herpesvirus (KSHV) K1 (Caldwell et al, 1998;Lee et al, 1998a;Katz et al, 2005). Studies of signaling by LMP2A (Longnecker et al, 1991;Caldwell et al, 1998;Swart et al, 2000) and K1 (Lee et al, 1998a;Lagunoff et al, 1999) in B cells suggests that these ITAMs are capable of signaling in a manner similar to traditional cellular ITAMs.…”

“…These proteins include EpsteinBarr virus (EBV) LMP2A, murine mammary tumor virus (MMTV) Env, and Kaposi's sarcoma-associated herpesvirus (KSHV) K1 (Caldwell et al, 1998;Lee et al, 1998a;Katz et al, 2005). Studies of signaling by LMP2A (Longnecker et al, 1991;Caldwell et al, 1998;Swart et al, 2000) and K1 (Lee et al, 1998a;Lagunoff et al, 1999) in B cells suggests that these ITAMs are capable of signaling in a manner similar to traditional cellular ITAMs. Additionally, nonhematopoietic cells expressing K1 and LMP2A are susceptible to transformation and in vivo tumor development (Lee et al, 1998b;Scholle et al, 2000;Prakash et al, 2002).…”

Kaposi's Sarcoma-Associated Herpesvirus-Encoded Latency-Associated Nuclear Antigen Modulates K1 Expression through Its cis -Acting Elements within the Terminal Repeats

“…K1 protein is related to the immunoglobulin receptor family and has similarity to the B-cell receptor (33). K1 is constitutively active through oligomerization via conserved, extracellular cysteine residues leading to the phosphorylation of the ITAM and recruitment of the major B-cell kinase Syk for the phosphorylation of cellular proteins, mobilization of intracellular calcium, and activation of transcription factors such as NFAT and AP-1 (21,27,28,30,33,38,58).…”

“…K1 protein is related to the immunoglobulin receptor family and has similarity to the B-cell receptor (33). K1 is constitutively active through oligomerization via conserved, extracellular cysteine residues leading to the phosphorylation of the ITAM and recruitment of the major B-cell kinase Syk for the phosphorylation of cellular proteins, mobilization of intracellular calcium, and activation of transcription factors such as NFAT and AP-1 (21,27,28,30,33,38,58). The amino-terminal domain of K1 interacts with the chain of the B-cell receptor complex leading to the inhibition of intracellular transport of B-cell receptor and resulting in retaining B-cell receptor complexes in the endoplasmic reticulum, preventing their intracellular transport to the cell surface which implies a role for K1 in the survival of KSHV-infected cells (26,30,32).…”

“…receptor tyrosine-based activation motif (ITAM) capable of activating signaling pathways (26,33,34,57,61). K1 protein is related to the immunoglobulin receptor family and has similarity to the B-cell receptor (33).…”