Degradation of the Epstein-Barr virus latent membrane protein 1 (LMP1) by the ubiquitin-proteasome pathway - Targeting via ubiquitination of the N-terminal residue

The latent membrane protein 1 (LMP1) of the Epstein-Barr virus is a constit utively active receptor essential for B lymphocyte transformation by the Ep stein-Barr virus. It is a short-lived protein, but the proteolytic pathway involved in its degradation is not known. The ubiquitin pathway is a major system for specific protein degradation in eukaryotes. Most plasma membrane substrates of the pathway are internalized upon ubiquitination and deliver ed for degradation in the lysosome/vacuole. Here we show that LMP1 is a sub strate of the ubiquitin pathway and is ubiquitinated both in vitro and in v ivo. However, in contrast to other plasma membrane substrates of the ubiqui tin system, it is degraded mostly by the proteasome and not by lysosomes. D egradation is independent of the single Lys residue of the protein; a lysin e-less mutant LMP1 is degraded in a ubiquitin- and proteasome-dependent man ner similar to the wild type protein, Degradation of both wild type and lys ine-less protein is sensitive to fusion of a Myc tag to the N terminus of L MP1, In addition, deletion of as few as 12 N-terminal amino acid residues s tabilizes the protein, These findings suggest that the first event in LMP1 degradation is attachment of ubiquitin to the N-terminal residue of the pro tein. We present evidence suggesting that phosphorylation is also required for degradation of LMP1.