Chimeric and Mutated Variants of LMP1: A Helpful Tool to Analyze the Structure-Function Relationship of a Pseudoreceptor

Epstein Barr virus (EBV) efficiently immortalizes human B cells in vitro generating lymphoblastoid cell lines (LCL) with indefinite lifespan. Latent membrane protein 1 (LMP1) belongs to a set of nine viral proteins expressed in vitro, five of which appear to be essential for B-cell immortalization (for review, see ref. 1 ). LMP1 is a membrane protein composed of a short cytoplasmic aminoterminus (24 residues), a transmembrane domain with six membrane-spanning segments separated by short reverse turns and a long cytoplasmic carboxy terminus (200 residues; see Fig. 1 ) ( 2 – 4 ). Genetic analysis has shown that LMP1 is indispensable but not sufficient for B-cell immortalization ( 5 – 7 ). In the last years the hypothesis was raised that LMP1 might act as a constitutively active receptor because it integrates into the plasma membrane and patches as an oligomer ( 3 , 8 ). This idea was further supported by LMP1’s ability to bind molecules involved in the signaling cascade of the TNF-receptor family members ( 9 – 12 ). In analogy to known receptors such as CD40 or TNF-R2, LMP1 activates cellular transcription factors of the NFкB and AP-1 family ( 13 – 15 ). Transcriptional activation of target genes is supposed to play a key role in EBV-mediated immortalization as well as LMP1-mediated oncogenicity.

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