CD40 and LMP-1 both signal from lipid rafts but LMP-1 assembles a distinct, more efficient signaling complex

Abstract

CD40, a member of the TNFR‐1 receptor family, shares several features with LMP‐1, an oncoprotein encoded by Epstein–Barr virus. CD40 and LMP‐1 activate transcription by binding to TRAFs, JAK3 and/or TRADD. CD40's association with CD40L activates signaling. However, LMP‐1 signals independently of a ligand but dependently on self‐association. We demonstrate that activated CD40 and LMP‐1 co‐localize in lipid rafts and recruit TRAF3 there, findings consistent with signals of CD40 and LMP‐1 being initiated from lipid rafts. To elucidate their signaling, we compared requirements for their aggregation and subcellular localization. Targeting CD40's monomeric C‐terminal signaling domain to lipid rafts activates signaling, as does rendering it trimeric. Addition of both modifications supports signaling more efficiently. Parallel experiments with LMP‐1 indicate that targeting the monomeric C‐terminal signaling domain of LMP‐1 to lipid rafts activates signaling, but trimerizing it does not. Fusing LMP‐1's N‐terminus and membrane‐spanning domains to CD40's C‐terminus supports signaling more efficiently than CD40 plus ligand or CD40's trimerized and/or localized derivatives. An activity of LMP‐1's N‐terminus and membrane‐spanning domains other than trimerization must contribute to its efficient signaling.