Activation of lymphocyte signaling by the R1 protein of rhesus monkey rhadinovirus

Rhesus monkey rhadinovirus (RRV) is a gamma-2 herpesvirus that exhibits a c onsiderable degree of similarity to the human Kaposi's sarcoma-associated h erpesvirus (KSHV). The R1 protein of RRV is distantly related to the K1 pro tein of KSHV, and R1, like K1, can contribute to cell growth transformation . In this study we analyzed the ability of the cytoplasmic tail of R1 to fu nction as a signal transducer. The cytoplasmic domain of the R1 protein con tains several tyrosine residues whose phosphorylation is induced in cells e xpressing Syk kinase. Expression of a CD8 chimera protein containing the ex tracellular and transmembrane domains of CD8 fused to the cytoplasmic domai n of R1 mobilized intracellular calcium and induced cellular tyrosine phosp horylation in B cells upon stimulation with anti-CD8 antibody. None of the CD8-R1 cytoplasmic deletion mutants tested were able to mobilize intracellu lar calcium or to induce tyrosine phosphorylation to a significant extent u pon addition of anti-CD8 antibody. Expression of wild-type R1 protein activ ated nuclear factor of activated T lymphocytes (NFAT) eightfold in Il cells in the absence of antibody stimulation; expression of the CD8-R1C chimera strongly induced NFAT activity (60-fold) but only upon the addition of anti -CD8 antibody. We conclude that the cytoplasmic domain of R1 is capable of transducing signals that elicit B-lymphocyte activation events. The signal- inducing properties of R1 appear to be similar to those of K1 but differ in that the required sequences are distributed over a much longer stretch of the cytoplasmic domain (>150 amino acids). In addition, the induction of ca lcium mobilization was considerably longer in duration and stronger with R1 than with K1.