A mutation in Epstein-Barr virus LMP2A reveals a role for phospholipase D in B-Cell antigen receptor trafficking.

Epstein-Barr virus (EBV) latent infection of B cells blocks the interrelated signaling and antigen-trafficking functions of the BCR through the activity of its latent membrane protein 2A (LMP2A). At present, the molecular mechanisms by which LMP2A exerts its control of BCR functions are only poorly understood. Earlier studies showed that in B cells expressing LMP2A containing a tyrosine mutation at position 112 in its cytoplasmic domain (Y112-LMP2A), the BCR could initiate signaling but could not properly traffic antigen for processing. Here, we show that BCR signaling in Y112-LMP2A-expressing cells is attenuated with a reduction in both the degree and duration of phosphorylation of key components of the BCR signaling cascade including Syk, BLNK, PI3K, and Btk. Notably, Y112-LMP2A expression completely blocked the BCR-induced activation of phospholipase D (PLD), a lipase implicated in the intracellular trafficking of a variety of surface receptors. We show that blocking PLD activity, by expressing Y112-LMP2A, treating cells with the PLD inhibitor 1-butanol or reducing PLD expression by siRNA, blocked BCR trafficking to class II-containing compartments. Moreover, Y112-LMP2A expression blocked the recruitment of phosphorylated forms of the downstream BCR signaling components, Erk and JNK, through both PLD-dependent and PLD-independent mechanisms. Thus, the investigation of the mechanism by which Y112-LMP2A blocks BCR function revealed an essential role for PLD in BCR trafficking for antigen processing.

Isolation of lipid rafts from B lymphocytes.

Recent advances in cell biology have provided evidence that the plasma membrane is not a homogeneous lipid bilayer but rather contains within it sphingolipid- and cholesterol-rich membrane microdomains, termed lipid rafts, which serve as platforms for both receptor signaling and trafficking. In B lymphocytes lipid rafts appear to play a key role in the initiation of B-cell antigen receptor (BCR) signaling. Current methods to isolate lipid rafts rely on the relative detergent insolubility of lipid rafts as compared to the nonraft, glycerophospholipid bilayer. Here a method to isolate and characterize lipid rafts from B lymphocytes is described. Particular emphasis is given to the potential artifacts inherent in current procedures that rely on detergents to isolate lipid rafts and alternative technologies that may circumvent these.