gp140, the EBV/C3d receptor (CR2) of human B lymphocytes, is involved in cell‐free phosphorylation of p120, a nuclear ribonucleoprotein

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Abstract
Gp140, the EBV/C3d receptor (EBV/C3dR; CR2), is a membrane site involved in human B cell regulation. Cross‐linking of this receptor on the cell surface by its specific ligands led to the enhancement of B cell proliferation in synergy with T cell factors. In vitro activation of human peripheral B lymphocytes by cross‐linking membrane immunoglobulins with anti‐μ antibody induced EBV/C3dR phosphorylation. These studies were pursued by analyzing cell‐free phosphorylation of EBV/C3dR isolated from Raji cell fractions, and immobilized on OKB7, a monoclonal anti‐EBV/C3dR antibody. Three EBV/C3dR‐related antigens which could be cell‐free phosphorylated were detected: gp140, the EBV/C3dR, p130 and p120. gp140, the mature form of EBV/C3dR, was isolated from plasma membrane and from purified nuclei. p130 was identified as an intracellular intermediate of EBV/C3dR glycosylation, localized in low‐density microsomes. Phosphoamino acid analysis of EBV/C3dR allowed the detection of phosphotyrosine and phosphoserine residues. These data suggest that EBV/C3dR could carry an autophosphorylation activity and could be associated to serine kinases. Using polyclonal anti‐p120 antibody and anti‐120 kDa nuclear ribocucleoprotein monoclonal antibody (mAb), p120 was identified as a nuclear ribonucleoprotein antigenically not related to EBV/C3dR. Detection of p120 on EBV/C3dR, immobilized on OKB7, was due to interactions between both antigens, instead of anti‐EBV/C3dR mAb cross‐reactivity with p120. Cell‐free phosphorylation of p120 was under the control of EBV/C3dR. However, it is not yet established whether other nuclear or membrane components were involved in the control of p120 cell‐free phosphorylation by EBV/C3dR. From the data presented herein, we propose that phosphorylation of a 120‐kDa nuclear ribonucleoprotein by EBV/C3dR‐associated kinases could represent a crucial step in in vivo regulation of human B cell activation.