Activation of Lymphocyte Signaling by the R1 Protein of Rhesus Monkey Rhadinovirus

ABSTRACT Rhesus monkey rhadinovirus (RRV) is a gamma-2 herpesvirus that exhibits a considerable degree of similarity to the human Kaposi's sarcoma-associated herpesvirus (KSHV). The R1 protein of RRV is distantly related to the K1 protein 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 function as a signal transducer. The cytoplasmic domain of the R1 protein contains several tyrosine residues whose phosphorylation is induced in cells expressing Syk kinase. Expression of a CD8 chimera protein containing the extracellular and transmembrane domains of CD8 fused to the cytoplasmic domain of R1 mobilized intracellular calcium and induced cellular tyrosine phosphorylation in B cells upon stimulation with anti-CD8 antibody. None of the CD8-R1 cytoplasmic deletion mutants tested were able to mobilize intracellular calcium or to induce tyrosine phosphorylation to a significant extent upon addition of anti-CD8 antibody. Expression of wild-type R1 protein activated nuclear factor of activated T lymphocytes (NFAT) eightfold in B 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 calcium mobilization was considerably longer in duration and stronger with R1 than with K1.

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Antigen-induced B lymphocyte activation involves the p21ras and ras.GAP signaling pathway.

Journal of Experimental Medicine ◽

10.1084/jem.178.5.1765 ◽

1993 ◽

Vol 178 (5) ◽

pp. 1765-1769 ◽

Cited By ~ 50

Author(s):

A H Lazarus ◽

K Kawauchi ◽

M J Rapoport ◽

T L Delovitch

Keyword(s):

B Cells ◽

B Cell ◽

Signaling Pathway ◽

Tyrosine Phosphorylation ◽

Cell Activation ◽

B Lymphocyte ◽

Lymphocyte Activation ◽

Nucleotide Exchange ◽

Western Blots ◽

B Lymphoma

Ligation of a B lymphocyte surface immunoglobulin (sIg) antigen receptor (AgR) by its specific Ag ligand initiates a signaling pathway that culminates in B cell activation. However, many events of this pathway have not been elucidated. Here we present three novel findings that demonstrate directly that AgR-mediated signaling in B cells functions by the p21ras/ras.GAP-dependent pathway. First, stimulation of TA3 7.9 Ag-specific murine B lymphoma cells for 2 min with either Ag or F(ab')2 anti-IgM induces p21ras activation as measured by an increase in the GTP/GDP ratio of its bound nucleotides. This activation of p21ras does not occur via a change in its guanine nucleotide exchange rate. Second, Ag stimulation results in the inhibition of activity of p120 ras.GAP, a protein that regulates p21ras activation. Tyrosine phosphorylation of ras.GAP occurs within 1 min after Ag stimulation but is no longer detectable at 20 min after stimulation, at which time ras.GAP activity remains inhibited. Thus, tyrosine phosphorylation of ras.GAP is not required for the inhibition of its activity. Third, despite the role proposed for a ras.GAP-associated p190 protein in the control of ras.GAP activity in B cells, p190 was not detectable either in anti-ras.GAP immunoprecipitates of [35S]methionine labeled lysates of Ag-stimulated or -unstimulated 7.9 cells or as a tyrosine phosphoprotein in Western blots of anti-ras.GAP immunoprecipitates of Ag-stimulated 7.9 cell lysates. Inasmuch as the TA3 7.9 B lymphoma is representative of a mature, sIgM-bearing B cell, our observations raise the intriguing possibility that the capacity of p190 to associate with ras.GAP and regulate the activities of ras.GAP and p21ras in a B cell is dependent on the stage of differentiation of the B cell.

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Role of C'3 and Fc receptors in B-lymphocyte activation.

Journal of Experimental Medicine ◽

10.1084/jem.141.3.647 ◽

1975 ◽

Vol 141 (3) ◽

pp. 647-663 ◽

Cited By ~ 48

Author(s):

G Möller ◽

A Coutinho

Keyword(s):

B Cells ◽

B Cell ◽

Polyclonal Antibody ◽

B Lymphocyte ◽

Fc Receptors ◽

Lymphocyte Activation ◽

Antibody Synthesis ◽

B Lymphocyte Activation ◽

Cell Induction

Attempts were made to identify the non-Ig lymphocyte receptor responsible for B-cell induction by antigen and polyclonal B-cell activators (PBA). As a first step, the role of C'3 and Fc receptors was analyzed. It was shown that complement could be fixed onto B cells to such an extent that the lymphocytes could not bind complement-coated red cells, but this did not result in induction of polyclonal antibody synthesis, nor did it inhibit the lymphocytes response to PBA. However, the C'3 receptros possessed a passive focussing role in the induction of polyclonal antibody responses. Thus, PBA that had fixed complement activated polyclonal responses at lower concentrations than the same substances that had not fixed complement. Most likely the dual binding of PBA molecules to B cells by the PBA and the C'3 receptors caused more PBA molecules to be bound to each cell. However, the focussing function of the C'3 receptors was several orders of magnitude smaller than that of the Ig receptors. Analogous studies were carried out with Fc receptors. Binding of different types of antigen-antibody complexes did not cause activation of polyclonal or specific antibody synthesis, nor did it significantly interfere with induction of antibody synthesis by PBA substances.

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